Remove initialization from PendingBackendStats

9a8dd2c has added an initialization to PendingBackendStats, which
has been causing compilation warnings in the buildfarm.  This code does
not strictly require it as PendingBackendStats is always initialized
with memset(0), so let's remove it.

Per report from multiple buildfarm members, like ayu and batfish, via
Tom Lane.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/1870853.1741749264@sss.pgh.pa.us

Author: michaelpq

src/backend/utils/activity/pgstat_backend.c

@@ -36,7 +36,7 @@
  * reported within critical sections so we use static memory in order to avoid
  * memory allocation.
  */
-static PgStat_BackendPending PendingBackendStats = {0};
+static PgStat_BackendPending PendingBackendStats;
 
 /*
  * WAL usage counters saved from pgWalUsage at the previous call to