pgsql: Don't try to run clauseless index scans on index types that don't

Log Message:
-----------
Don't try to run clauseless index scans on index types that don't support
it. Per report from Marinos Yannikos.

Tags:
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REL7_3_STABLE

Modified Files:
--------------
pgsql/src/backend/optimizer/path:
indxpath.c (r1.124.2.1 -> r1.124.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.124.2.1&r2=1.124.2.2)