Manage:Import feature flag cleanup

We currently have a few feature flags in ~"group::import". Some of them have been enabled by default for a while.

As a first step, let's verify if feature flags that are enabled by default, can be removed.

Enabled by default

feature flag	code references	rollout issue	DRI	Notes
project_export_as_ndjson	yes	-		This seems to be not a ~"group::import" feature anymore (see !26995 (merged))
project_import_ndjson	yes	-		This seems to be not a ~"group::import" feature anymore (see !27206 (merged))
validate_import_decompressed_archive_size	yes	#282245 (closed)		Should remain OFF, there is an issue that needs to be addressed before it can be fully turned on as per #282245 (comment 448040848)

Next, we should look into feature flags that are currently disabled by default and see if they are still relevant and if some of them should be enabled by default.

Disabled by default

feature flag	code references	rollout issue	DRI	Notes
bitbucket_server_user_mapping_by_username	yes	N/A	@.luke	This has been changed to an ops flag in !121335 (merged) and we will not roll it out or remove it. This was a Community contribution see community !36885 (merged)
github_importer_lower_per_page_limit	yes	-	@georgekoltsov	operational feature flag that should remain as it supports big project migrations in case of GitHub API instability, see https://docs.gitlab.com/ee/user/project/import/github.html#alternative-way-to-import-notes-and-diff-notes
github_importer_single_endpoint_notes_import	yes	-	@georgekoltsov	operational feature flag that should remain as it supports big project migrations in case of GitHub API instability, see https://docs.gitlab.com/ee/user/project/import/github.html#alternative-way-to-import-notes-and-diff-notes
remove_legacy_github_client	yes	#351658 (closed)	@georgekoltsov	Can be removed as per #282245 (comment 639489578)
import_project_from_remote_file_s3	yes	#350571 (closed)

Note: As soon as a feature flag has been removed from the codebase, let's also make sure that we clean up our database records via chatops (see https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up)