Clean up syscache so that recursive invocation is safe, and remove error

Clean up syscache so that recursive invocation is safe, and remove error
message about recursive use of a syscache.  Also remove most of the
specialized indexscan routines in indexing.c --- it turns out that
catcache.c is perfectly able to perform the indexscan for itself,
in fact has already looked up all the information needed to do so!
This should be faster as well as needing far less boilerplate code.