Remove unnecessary mux

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This merge request updates test configuration across multiple GitLab provider test files. The main change is replacing testAccProtoV6MuxProviderFactories with testAccProtoV6ProviderFactories throughout the test suite, which appears to be switching from a multiplexed provider factory to a simpler provider factory setup.

Additionally, the changes include minor code cleanup such as:

• Fixing comment formatting (changing //lintignore to // lintignore)
• Removing extra blank lines to improve code consistency
• Moving a provider factory configuration line to the proper location in one test file

These changes affect test files for various GitLab resources including current user data, group access tokens, project environments, merge requests, protected branches, and many other GitLab features. The updates appear to be part of a broader effort to standardize the test infrastructure and improve code formatting consistency across the codebase.