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 850b9218c8.
Discussion: https://postgr.es/m/ajZz_sVJVX7pmPHo@alvherre.pgsql
Backpatch-through: 15-18
fe464e9e68