Remove serialized bitbucket importer and feature flag

.rubocop_todo/layout/argument_alignment.yml

.rubocop_todo/layout/array_alignment.yml

.rubocop_todo/layout/first_hash_element_indentation.yml

.rubocop_todo/layout/line_length.yml

.rubocop_todo/lint/redundant_cop_disable_directive.yml

.rubocop_todo/lint/unused_method_argument.yml

.rubocop_todo/rails/time_zone.yml

.rubocop_todo/rspec/context_wording.yml

.rubocop_todo/rspec/instance_variable.yml

.rubocop_todo/rspec/named_subject.yml

.rubocop_todo/rspec/return_from_stub.yml

.rubocop_todo/rspec/verified_doubles.yml

.rubocop_todo/style/guard_clause.yml

.rubocop_todo/style/inline_disable_annotation.yml

...feature_flags/development/bitbucket_parallel_importer.yml

doc/development/bitbucket_cloud_importer.md

doc/user/project/import/bitbucket.md

lib/gitlab/bitbucket_import/importer.rb

lib/gitlab/bitbucket_import/importers/repository_importer.rb

lib/gitlab/import_sources.rb