Remove stdbool workaround in sepgsql

Since we now use stdbool.h in c.h, this workaround breaks the build and
is no longer necessary, so remove it.  (Technically, there could be
platforms with a 4-byte bool in stdbool.h, in which case we would not
include stdbool.h in c.h, and so the old problem that caused this
workaround would reappear.  But this combination is not known to happen
on the range of platforms where sepgsql can be built.)

contrib/sepgsql/label.c

@@ -12,14 +12,6 @@
 
 #include <selinux/label.h>
 
-/*
- * <selinux/label.h> includes <stdbool.h>, which creates an incompatible
- * #define for bool.  Get rid of that so we can use our own typedef.
- * (We don't care if <stdbool.h> redefines "true"/"false"; those are close
- * enough.)
- */
-#undef bool
-
 #include "access/heapam.h"
 #include "access/htup_details.h"
 #include "access/genam.h"