Remove unneeded probes from configure and meson

7d854bdc5b has removed two symbols from pg_config.h.in.  This file is
automatically generated.  The correct cleanup needs to be done in the
build scripts, instead.  autoheader produces now a consistent
pg_config.h.in, without the symbols that were removed in the previous
commit.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1193764.1767573683@sss.pgh.pa.us

configure

@@ -13669,7 +13669,7 @@ fi
 ## Header files
 ##
 
-for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h mbarrier.h sys/epoll.h sys/event.h sys/personality.h sys/prctl.h sys/procctl.h sys/signalfd.h sys/ucred.h termios.h uchar.h ucred.h xlocale.h
+for ac_header in copyfile.h execinfo.h getopt.h ifaddrs.h sys/epoll.h sys/event.h sys/personality.h sys/prctl.h sys/procctl.h sys/signalfd.h sys/ucred.h termios.h uchar.h ucred.h xlocale.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
 ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"

configure.ac

@@ -1501,12 +1501,10 @@ AC_SUBST(UUID_LIBS)
 ##
 
 AC_CHECK_HEADERS(m4_normalize([
-	atomic.h
 	copyfile.h
 	execinfo.h
 	getopt.h
 	ifaddrs.h
-	mbarrier.h
 	sys/epoll.h
 	sys/event.h
 	sys/personality.h

meson.build

@@ -2626,13 +2626,11 @@ endif
 # XXX: Might be worth conditioning some checks on the OS, to avoid doing
 # unnecessary checks over and over, particularly on windows.
 header_checks = [
-  'atomic.h',
   'copyfile.h',
   'crtdefs.h',
   'execinfo.h',
   'getopt.h',
   'ifaddrs.h',
-  'mbarrier.h',
   'strings.h',
   'sys/epoll.h',
   'sys/event.h',