pgsql: Remove unused tuple expansion and copying functions

Remove unused tuple expansion and copying functions

minimal_expand_tuple() has been unused since 4da597edf1b and
heap_expand_tuple() since 20d3fe9009d, which in turn left the static
expand_tuple() unreachable. heap_copytuple_with_tuple() has had no
callers since 141fd1b66ce. Remove them.

Two regression test comments naming expand_tuple() are updated to
refer to the code that handles missing attributes nowadays.

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Reported-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Author: Aleksander Alekseev <aleksander(at)tigerdata(dot)com>
Reviewed-by: Kyle Snavely <kjsnavely(at)gmail(dot)com>
Discussion: https://postgr.es/m/gf6hokynqjp6g7o4zh6yy7ekbdbig5cnnd6ilk6iw3aodilj43%405x7xrz52jhve

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/81b2179b492e3af891544813b7e22a42386b34d0

Modified Files
--------------
src/backend/access/common/heaptuple.c | 290 -----------------------------
src/include/access/htup_details.h | 3 -
src/test/regress/expected/fast_default.out | 4 +-
src/test/regress/sql/fast_default.sql | 4 +-
4 files changed, 4 insertions(+), 297 deletions(-)