Revert "glsl: Set location on structure-split sampler uniform variables"

This reverts commit 9e0c744f, which
regressed dEQP-GLES2.functional.uniform_api.random.3.  It turns out
that the newly produced location is meaningless and impossible to
consume by drivers that want to look at gl_uniform_storage, so it's
probably better to leave it unset (0) than a number that looks usable.

Leave a tombstone^Wcomment to discourage the next person from making
the obvious looking fix.

See the next commit for a longer description of the problem.

This breaks tests/spec/glsl-1.10/execution/samplers/uniform-struct
on i965, which was originally fixed by the revert.  The next commit
will fix it again.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 47303b46)