Remove redundant DataBoundConstructor / DataBoundSetter on GitLabConnectionConfig (#1937)

The object is already an Extension.  It does not need a DataBoundConstructor or DataBoundSetter.

Testing done:

* Automated tests pass, including the configuration as code automated test
* Interactive testing checked the system configuration page by adding
  a GitLab connection with a legacy API token and pressing the Test
  Connection button.  The test passed with the legacy API token.
* Interactive testing checked the system configuration page by adding
  a GitLab connection with a fine-grained API token and pressing the Test
  Connection button.  The test failed with the fine-grained API token.  I
  assume that I had incorrect definitions on the fine-grained API token