Remove redundant pg_unreachable() after elog(ERROR) from ExecWaitStmt()

Remove redundant pg_unreachable() after elog(ERROR) from ExecWaitStmt()

elog(ERROR) never returns.  Compilers don't always understand this.  So,
sometimes, we have to append pg_unreachable() to keep the compiler quiet
about returning from a non-void function without a value.  But
pg_unreachable() is redundant for ExecWaitStmt(), which is void.

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Author: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/8d72a2b3-7423-4a15-a981-e130bf60b1a6%40eisentraut.org
Discussion: https://postgr.es/m/CABPTF7UcuVD0L-X%3DjZFfeygjPaZWWkVRwtWOaJw2tcXbEN2xsA%40mail.gmail.com