pgsql: Remove a couple of unnecessary calls of CreateCacheMemoryContext.

Log Message:
-----------
Remove a couple of unnecessary calls of CreateCacheMemoryContext.  These
probably got there via blind copy-and-paste from one of the legitimate
callers, so rearrange and comment that code a bit to make it clearer that
this isn't a necessary prerequisite to hash_create.  Per observation
from Robert Haas.

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        predtest.c (r1.27 -> r1.28)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c?r1=1.27&r2=1.28)
    pgsql/src/backend/parser:
        parse_oper.c (r1.109 -> r1.110)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.109&r2=1.110)
    pgsql/src/backend/utils/cache:
        relcache.c (r1.293 -> r1.294)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c?r1=1.293&r2=1.294)
        ts_cache.c (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/ts_cache.c?r1=1.9&r2=1.10)
        typcache.c (r1.29 -> r1.30)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/typcache.c?r1=1.29&r2=1.30)