Remove obsolete code in importers and remove new_import_ui feature flag"

What does this MR do?

See #220609 (closed) for details

In general it:

• removes :new_import_ui feature flag
• makes new ui (hidden behind flag) new default
• refactors github/gitea import controller to use same code as other importers controllers
• removes old templates

Out of scope of this MR, but mentioned in #220609 (closed):

• removing incompatible_repos support (bitbucket cloud still works with mercurial ATM :( )
• removing old js (blocked by removing google code importer in #219049 (closed))
• updating docs screenshots (will be done in separate MR)

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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