pgsql: Re-introduce pgstat_drop_entry(), keeping ABI compatibility

Re-introduce pgstat_drop_entry(), keeping ABI compatibility

This routine acts as a wrapper of a new pgstat_drop_entry_ext(), used in
the core code with a missing_ok argument.

This includes an update of .abi-compliance-history, removing the latest
entry that has documented the change of pgstat_drop_entry(). This
change is applied across v15~v18. HEAD keeps pgstat_drop_entry() as
single entry point, with the new missing_ok.

Per discussion with Álvaro Herrera and Lukas Fittl. This is a follow-up
of 850b9218c8e4.

Discussion: https://postgr.es/m/ajZz_sVJVX7pmPHo@alvherre.pgsql
Backpatch-through: 15-18

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e34b1ff5d89494810a870861505f1b29a731f1ce

Modified Files
--------------
.abi-compliance-history | 9 ---------
src/backend/utils/activity/pgstat_function.c | 4 ++--
src/backend/utils/activity/pgstat_replslot.c | 4 ++--
src/backend/utils/activity/pgstat_shmem.c | 17 ++++++++++++++---
src/backend/utils/activity/pgstat_xact.c | 8 ++++----
src/include/utils/pgstat_internal.h | 5 +++--
6 files changed, 25 insertions(+), 22 deletions(-)