[TASK] Remove obsolete files

Some files are found in extensions that were submodules a while ago:
* The .gitignore file of linkvalidator is identical to the main
  .gitignore file and can be removed
* The .gitignore file together with the UnitTests.xml of extbase
  is covered by core UnitTests.xml

Change-Id: Ief188d10d2caa428d10f67b6b3e7ab1cc0d768f6
Resolves: #52756
Releases: 6.2
Reviewed-on: https://review.typo3.org/24703
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters