pgsql: Clean up in the wake of TupleDescGetSlot() removal / 10763358c3f

Clean up in the wake of TupleDescGetSlot() removal / 10763358c3f.

The previous commit wasn't careful enough to remove all traces of
TupleDescGetSlot().

Besides fixing the oversight of not removing TupleDescGetSlot()'s
declaration, this also removes FuncCallContext->slot. That was
documented to be for use in combination with TupleDescGetSlot(), a
cursory search over extensions finds no users, and there doesn't seem
to be convincing reasons to keep it around. If we later in the v12
release cycle find users, we can re-consider this part of the commit.

Reported-By: Michael Paquier
Discussion: https://postgr.es/m/20180926000413.GC1659@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/27e082b0c6e564facfbf54b56090fdcc4bf44cca

Modified Files
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doc/src/sgml/xfunc.sgml | 8 --------
src/backend/utils/fmgr/funcapi.c | 16 ----------------
src/include/funcapi.h | 11 -----------
3 files changed, 35 deletions(-)