Remove old docker settings

This will stop them from validating in the config,
since we are removing them from the build servers.

We have never supported or documented the support for pinning to a specific version,
so I think this should be safe.

Author: ericholscher

readthedocs/settings/base.py

@@ -370,33 +370,6 @@ def USE_PROMOS(self):  # noqa
     DOCKER_DEFAULT_VERSION = 'latest'
     DOCKER_IMAGE = '{}:{}'.format(DOCKER_DEFAULT_IMAGE, DOCKER_DEFAULT_VERSION)
     DOCKER_IMAGE_SETTINGS = {
-        'readthedocs/build:1.0': {
-            'python': {
-                'supported_versions': [2, 2.7, 3, 3.4],
-                'default_version': {
-                    2: 2.7,
-                    3: 3.4,
-                },
-            },
-        },
-        'readthedocs/build:2.0': {
-            'python': {
-                'supported_versions': [2, 2.7, 3, 3.5],
-                'default_version': {
-                    2: 2.7,
-                    3: 3.5,
-                },
-            },
-        },
-        'readthedocs/build:3.0': {
-            'python': {
-                'supported_versions': [2, 2.7, 3, 3.3, 3.4, 3.5, 3.6],
-                'default_version': {
-                    2: 2.7,
-                    3: 3.6,
-                },
-            },
-        },
         'readthedocs/build:4.0': {
             'python': {
                 'supported_versions': [2, 2.7, 3, 3.5, 3.6, 3.7],