Remove support for multiple Linux shell extensions

[TheOneRing]

With 6.0 we will retire the branded Linux repositories.
In the future the same pattern as on Windows will apply, the first ownCloud client, branded or unbranded, will open a standard local socket.
Then the vanilla shell extension (dolphin/nautilus) will connect to that socket.
If multiple clients are running locally, only the first one will have proper shell integration.

A user will have to use the default shell extension provided by their distribution.

• Hard-code ownCloud in https://github.com/owncloud/client/blob/master/src/gui/guiutility_unix.cpp#L28 , same as we have on Windows https://github.com/owncloud/client/blob/master/src/gui/guiutility_win.cpp#L28