Remove unused feature flags

Signed-off-by: Rémy Coutable <remy@rymai.me>

config/feature_flags/development/usage_data_static_site_editor_commits.yml

----
-name: usage_data_static_site_editor_commits
-introduced_by_url:
-rollout_issue_url:
-milestone:
-type: development
-group: group::static_site_editor
-default_enabled: false

config/feature_flags/development/usage_data_static_site_editor_merge_requests.yml

----
-name: usage_data_static_site_editor_merge_requests
-introduced_by_url:
-rollout_issue_url:
-milestone:
-type: development
-group: group::static_site_editor
-default_enabled: false