Don't define FRONTEND for libpq

Not needed anymore after 7143b3e821.

Discussion: https://postgr.es/m/20220820194550.725755r6fj2ro3rx@awork3.anarazel.de

src/interfaces/libpq/Makefile

@@ -22,7 +22,7 @@ NAME= pq
 SO_MAJOR_VERSION= 5
 SO_MINOR_VERSION= $(MAJORVERSION)
 
-override CPPFLAGS :=  -DFRONTEND -I$(srcdir) $(CPPFLAGS) -I$(top_builddir)/src/port -I$(top_srcdir)/src/port
+override CPPFLAGS :=  -I$(srcdir) $(CPPFLAGS) -I$(top_builddir)/src/port -I$(top_srcdir)/src/port
 ifneq ($(PORTNAME), win32)
 override CFLAGS += $(PTHREAD_CFLAGS)
 endif

src/tools/msvc/Mkvcbuild.pm

@@ -263,7 +263,6 @@ sub mkvcbuild
 
 	$libpq = $solution->AddProject('libpq', 'dll', 'interfaces',
 		'src/interfaces/libpq');
-	$libpq->AddDefine('FRONTEND');
 	$libpq->AddIncludeDir('src/port');
 	$libpq->AddLibrary('secur32.lib');
 	$libpq->AddLibrary('ws2_32.lib');