pgsql: Remove the obsolete WITH clause of CREATE FUNCTION.

Remove the obsolete WITH clause of CREATE FUNCTION.

This clause was superseded by SQL-standard syntax back in 7.3.
We've kept it around for backwards-compatibility purposes ever since;
but 15 years seems like long enough for that, especially seeing that
there are undocumented weirdnesses in how it interacts with the
SQL-standard syntax for specifying the same options.

Michael Paquier, per an observation by Daniel Gustafsson;
some small cosmetic adjustments to nearby code by me.

Discussion: https://postgr.es/m/20180115022748.GB1724@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4971d2a32209118ebbdc6611341b89901e340902

Modified Files
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doc/src/sgml/ref/create_function.sgml | 36 -----------
src/backend/commands/functioncmds.c | 109 +++++++++-------------------------
src/backend/nodes/copyfuncs.c | 5 +-
src/backend/nodes/equalfuncs.c | 3 +-
src/backend/parser/gram.y | 14 ++---
src/include/nodes/parsenodes.h | 3 +-
6 files changed, 38 insertions(+), 132 deletions(-)