Build: cleanup VCS support to remove unused methods

[humitos]

Since the deploy that implements the Celery Handlers, we are always starting the build from a fresh and cleaned environment. No cache is used at all.

So, we should be able to delete all the code related to update the repository by calling git fetch and similar. Take a look at

readthedocs.org/readthedocs/vcs_support/backends/git.py

Lines 154 to 176 in b5a05f2

	def fetch(self):
	# --force lets us checkout branches that are not fast-forwarded
	# https://github.com/readthedocs/readthedocs.org/issues/6097
	cmd = ['git', 'fetch', 'origin',
	'--force', '--tags', '--prune', '--prune-tags']
	if self.use_shallow_clone():
	cmd.extend(['--depth', str(self.repo_depth)])
	if self.verbose_name and self.version_type == EXTERNAL:
	if self.project.git_provider_name == GITHUB_BRAND:
	cmd.append(
	GITHUB_PR_PULL_PATTERN.format(id=self.verbose_name)
	)
	if self.project.git_provider_name == GITLAB_BRAND:
	cmd.append(
	GITLAB_MR_PULL_PATTERN.format(id=self.verbose_name)
	)
	code, stdout, stderr = self.run(*cmd)
	return code, stdout, stderr

as example.