pgstat: remove some superflous comments from pgstat.h.

pgstat: remove some superflous comments from pgstat.h.

These would all need to be rephrased when moving to shared memory stats, but
since they don't provide actual information right now, remove them instead.

The comments for PgStat_Msg* are left in, because they will all be removed as
part of the shared memory stats patch.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de