PG-174: Code cleanup. (#310)

pg_stat_monitor is a bit longer; therefore, it requires some code cleanup.
Therefore I decided to turn these tasks into multiple commits and PR to avoid
various changes in one PR. This will ease the review and Q/A process.
In this commit, I have done these tasks.

1 - Fixing compilation issue, cause by previous commit, where we removed
the benchmarking code. It was causing problem for PostgreSQL-12.

pg_stat_monitor.c

@@ -394,7 +394,7 @@ pgss_post_parse_analyze(ParseState *pstate, Query *query, JumbleState *jstate)
 				   jstate,		/* JumbleState */
 				   PGSS_PARSE); /* pgssStoreKind */
 }
-#elif
+#else
 
 /*
  * Post-parse-analysis hook: mark query with a queryId