pgsql: Remove lappend_cell...() family of List functions.

Remove lappend_cell...() family of List functions.

It seems worth getting rid of these functions because they require the
caller to retain a ListCell pointer into a List that it's modifying,
which is a dangerous practice with the new List implementation.
(The only other List-modifying function that takes a ListCell pointer
as input is list_delete_cell, which nowadays is preferentially used
via the constrained API foreach_delete_current.)

There was only one remaining caller of these functions after commit
2f5b8eb5a, and that was some fairly ugly GEQO code that can be much
more clearly expressed using a list-index variable and list_insert_nth.
Hence, rewrite that code, and remove the functions.

Discussion: https://postgr.es/m/26193.1563228600@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c245776906b065fcd59831a25c3b24ad3ddcd849

Modified Files
--------------
src/backend/nodes/list.c               | 47 ----------------------------------
src/backend/optimizer/geqo/geqo_eval.c | 17 +++++-------
src/include/nodes/pg_list.h            |  4 ---
3 files changed, 6 insertions(+), 62 deletions(-)