pgsql: Remove incorrect file reference in comment.

Remove incorrect file reference in comment.

Commit b7eda3e0e moved XidInMVCCSnapshot() from tqual.c into snapmgr.c,
but follow-up commit c91560def incorrectly updated this reference.  We
could fix it, but as pointed out by Daniel Gustafsson, 1) the reader can
easily find the file that contains the definition of that function, e.g.
by grepping, and 2) this kind of reference is prone to going stale; so
let's just remove it.

Back-patch to all supported branches.

Reviewed by Daniel Gustafsson.

Discussion: https://postgr.es/m/CAPmGK145VdKkPBLWS2urwhgsfidbSexwY-9zCL6xSUJH%2BBTUUg%40mail.gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7b6ba8c4315e5657fd90bad91c552fca58dc87fd

Modified Files
--------------
src/backend/storage/ipc/procarray.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)