Mamba: remove CONDA_USES_MAMBA feature flag

Mamba can be used via `build.tools.python: mambaforge` now.

This PR removes the `CONDA_USES_MAMBA` feature flag since we already contact
most of the users and opened a PR on their repositories with the config file
required for this change and we also sent an in-site notification to all those
projects still using it.

The message said this feature flag will be removed on May 1st. So, we are all
good to merge and deploy next week (May 3rd).

Author: humitos

readthedocs/doc_builder/python_environments.py

@@ -447,10 +447,6 @@ def conda_bin_name(self):
         """
         Decide whether use ``mamba`` or ``conda`` to create the environment.
 
-        Return ``mamba`` if the project has ``CONDA_USES_MAMBA`` feature and
-        ``conda`` otherwise. This will be the executable name to be used when
-        creating the conda environment.
-
         ``mamba`` is really fast to solve dependencies and download channel
         metadata on startup.
 
@@ -459,10 +455,6 @@ def conda_bin_name(self):
         # Config file using ``build.tools.python``
         if self.config.using_build_tools:
             return self.config.python_interpreter
-
-        # Config file using ``conda``
-        if self.project.has_feature(Feature.CONDA_USES_MAMBA):
-            return 'mamba'
         return 'conda'
 
     def _update_conda_startup(self):
@@ -473,8 +465,6 @@ def _update_conda_startup(self):
         independently the version of Miniconda that it has installed.
         """
         self.build_env.run(
-            # TODO: use ``self.conda_bin_name()`` once ``mamba`` is installed in
-            # the Docker image
             'conda',
             'update',
             '--yes',
@@ -485,19 +475,6 @@ def _update_conda_startup(self):
             cwd=self.checkout_path,
         )
 
-    def _install_mamba(self):
-        self.build_env.run(
-            'conda',
-            'install',
-            '--yes',
-            '--quiet',
-            '--name=base',
-            '--channel=conda-forge',
-            'python=3.7',
-            'mamba',
-            cwd=self.checkout_path,
-        )
-
     def setup_base(self):
         conda_env_path = os.path.join(self.project.doc_path, 'conda')
         os.path.join(conda_env_path, self.version.slug)
@@ -509,15 +486,6 @@ def setup_base(self):
             self._append_core_requirements()
             self._show_environment_yaml()
 
-        if all([
-                # The project has CONDA_USES_MAMBA feature enabled and,
-                self.project.has_feature(Feature.CONDA_USES_MAMBA),
-                # the project is not using ``build.tools``,
-                # which has mamba installed via asdf.
-                not self.config.using_build_tools,
-        ]):
-            self._install_mamba()
-
         self.build_env.run(
             self.conda_bin_name(),
             'env',
@@ -606,9 +574,6 @@ def _get_core_requirements(self):
             'pillow',
         ]
 
-        if self.project.has_feature(Feature.CONDA_USES_MAMBA):
-            conda_requirements.append('pip')
-
         # Install pip-only things.
         pip_requirements = [
             'recommonmark',

readthedocs/projects/models.py

@@ -1753,7 +1753,6 @@ def add_features(sender, **kwargs):
     CLEAN_AFTER_BUILD = "clean_after_build"
     UPDATE_CONDA_STARTUP = "update_conda_startup"
     CONDA_APPEND_CORE_REQUIREMENTS = "conda_append_core_requirements"
-    CONDA_USES_MAMBA = "conda_uses_mamba"
     ALL_VERSIONS_IN_HTML_CONTEXT = "all_versions_in_html_context"
     CACHED_ENVIRONMENT = "cached_environment"
     LIMIT_CONCURRENT_BUILDS = "limit_concurrent_builds"
@@ -1824,10 +1823,6 @@ def add_features(sender, **kwargs):
             CONDA_APPEND_CORE_REQUIREMENTS,
             _('Append Read the Docs core requirements to environment.yml file'),
         ),
-        (
-            CONDA_USES_MAMBA,
-            _('Uses mamba binary instead of conda to create the environment'),
-        ),
         (
             ALL_VERSIONS_IN_HTML_CONTEXT,
             _(