Revert "Prevent editing commits" feature

What does this MR do?

On &4983 (closed), we toyed with the idea of allowing editing commits on unprotected branches to support generating Changelog based on merge requests.

Rollout of the new changelog workflow was completed successfully &4983 (comment 631424441), so the feature is no longer required. It can be re-added later if necessary.

This MR:

• Reverts !51235 (merged)
• Reverts !49152 (merged)
• Makes db/migrate/20201203144655_add_allow_to_edit_commit_to_project_settings.rb no-op and ignores allow_editing_commit_messages. Issue to remove the column and ignore_column statement #336783 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.