pgsql: Remove now-dead code in StoreAttrDefault().

Remove now-dead code in StoreAttrDefault().

StoreAttrDefault() is no longer responsible for filling
attmissingval, so remove the code for that.

Get rid of RawColumnDefault.missingMode, too, as we no longer
need that to pass information around.

While here, clean up some sloppy coding in StoreAttrDefault(),
such as failure to use XXXGetDatum macros. These aren't bugs
but they're not good code either.

Reported-by: jian he <jian(dot)universality(at)gmail(dot)com>
Author: jian he <jian(dot)universality(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CACJufxHFssPvkP1we7WMhPD_1kwgbG52o=kQgL+TnVoX5LOyCQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4528768d98f83070d625207dcb526982fb18f1ce

Modified Files
--------------
src/backend/catalog/heap.c | 5 ++-
src/backend/catalog/pg_attrdef.c | 77 ++++------------------------------------
src/backend/commands/tablecmds.c | 8 ++---
src/include/catalog/heap.h | 1 -
src/include/catalog/pg_attrdef.h | 3 +-
5 files changed, 11 insertions(+), 83 deletions(-)