Revert "x2goclient: move to qmake4Hook"

This reverts commit 83406bc1, because
it broke the build.

x2goclient requires to be built with its top-level (hand coded) Makefile
(in accordance with upstream documentation). Invoking qmake directly on
the .pro file, without specifying a separate build tree, will overwrite
the Makefile and break the build.

For instance, there are no install rules in the .pro file. That exists
only in the Makefile.