Remove unused compiler flags from mapreduce Makefile

GPMapreduce isn't threaded so -pthreads is a noop causing annoying
warning messages in clang.

gpAux/extensions/gpmapreduce/Makefile

@@ -18,10 +18,6 @@ SRCDIR= src
 BUILDDIR= src
 endif
 
-ifneq ($(PORTNAME), win32)
-override CFLAGS += $(PTHREAD_CFLAGS) -pthread
-endif
-
 override CPPFLAGS += $(BLD_CPPFLAGS_SOLARIS_OLD_VERSIONS) -DFRONTEND -I$(SRCDIR) -I$(INCDIR) -I$(libpq_srcdir)
 override LDFLAGS  += -lyaml