Remove partial clone feature flag

What does this MR do?

It removes the gitaly_upload_pack_filter which was enabled by default and could be used to disable partial clone.

As far as I know the feature flag used to be documented when it was disabled by default, but its documentation was removed when it was enabled by default. We remove documentation mentioning the underlying Git config options, as it's not relevant anymore.

Some tests about what happens when the feature flag is disabled are removed.

This implements: gitaly#2874 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides

Availability and Testing

If people use partial clone too much or without thinking through the consequences of the commands, it could use a lot of resources on the server. It doesn't seem like this happened when the feature flag was enabled by default though.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done