Remove PgStat_BackendFunctionEntry
This structure included only PgStat_FunctionCounts, and removing it
facilitates some upcoming refactoring for pgstatfuncs.c to use more
macros rather that mostly-duplicated functions.
Author: Bertrand Drouvot
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/
11d531fe-52fc-c6ea-7e8e-
62f1b6ec626e@gmail.com