TestFiles::testFiles (clang): remove obsolete workaround

text/x-opencl-src was added to shared-mime-info 4 years ago in
https://gitlab.freedesktop.org/xdg/shared-mime-info/-/commit/4ab969c398671c519a10a601ad3eeb46f04361de

This MIME type is present in shared-mime-info version >= 1.9. Even
oldstable Debian "buster" has version 1.10. KDevelop's dependency
requirements don't allow building it on old distribution versions. So
there is no reason to keep the workaround.

This reverts commit 056932ba.