Revert "gsettings m4: check for .xml in src/builddir"

This reverts commit b3593693d918f0ae97094f6712d817180b8eea6a.

See: https://bugzilla.gnome.org/show_bug.cgi?id=712171#c3
See: https://bugzilla.gnome.org/show_bug.cgi?id=712171#c4

Tested using both srcdir == builddir and srcdir != builddir in hotssh.

Conflicts:
	m4macros/gsettings.m4

https://bugzilla.gnome.org/show_bug.cgi?id=712630