xfstests: Decomplicate quota setup in 050

xfstests: Decomplicate quota setup in 050

Only test 050 uses the _qsetup function, and it only uses the ID
part of it in the test. The attempts to link output files is useless
as there are no different output files used by the test.

Hence kill the _qsetup function, and move the code into test 050,
simplifying it down to the minimum needed.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Phil White <pwhite@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>