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.