Remove the git commit hash from the Linux build process

[pentix]

See #41 (comment)

[pentix]

I did some more research on this [1], [2] and found there is not really a cross-platform way to obtain the latest git revision before compiling qjournalctl. It's additionally difficult since git can be installed everywhere on a windows system and might not be available in %PATH% therefore the user would need to set it - just like the Qt and msvc path. I assume most users on Windows would use the precompiled binaries anyway, where the git revision would not be shown.

Also, it's probably "more fancy than a feature", i.e. the use is very limited, so I might as well just remove it in the linux build process.

[1] https://www.everythingfrontend.com/posts/app-version-from-git-tag-in-qt-qml.html

[2] https://stackoverflow.com/questions/27041573/print-git-hash-in-qt-as-macro-created-at-compile-time/27062550