Gemfile: Remove Gemfile.local support

[bastelfreak]

We don't use a Gemfile.local in this repository and dependabot refuse to parse it.

Dependabot encountered '1' error(s) during execution, please check the logs for more details.
+-------------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                                        Errors                                                                         |
+-------------------------------+-----------------------------------------------------------------------------------------------------------------------+
| Type                          | Details                                                                                                               |
+-------------------------------+-----------------------------------------------------------------------------------------------------------------------+
| dependency_file_not_parseable | {                                                                                                                     |
|                               |   "message": "Dependabot only supports uninterpolated string arguments to eval_gemfile. Got `\"#{__FILE__}.local\"`", |
|                               |   "file-path": "/Gemfile"                                                                                             |
|                               | }                                                                                                                     |
+-------------------------------+-----------------------------------------------------------------------------------------------------------------------+

[smortex]

Also remove it from .gitignore and LGTM!