rts: Drop redundant flags for libffi

These are now handled in the cabal file's include-dirs field.

rts/ghc.mk

@@ -489,15 +489,10 @@ endif
 endif
 
 # add CFLAGS for libffi
-ifeq "$(UseSystemLibFFI)" "YES"
-LIBFFI_CFLAGS = $(addprefix -I,$(FFIIncludeDir))
-else
-LIBFFI_CFLAGS =
-endif
 # ffi.h triggers prototype warnings, so disable them here:
-rts/Interpreter_CC_OPTS += -Wno-strict-prototypes $(LIBFFI_CFLAGS)
-rts/Adjustor_CC_OPTS    += -Wno-strict-prototypes $(LIBFFI_CFLAGS)
-rts/sm/Storage_CC_OPTS  += -Wno-strict-prototypes $(LIBFFI_CFLAGS)
+rts/Interpreter_CC_OPTS += -Wno-strict-prototypes
+rts/Adjustor_CC_OPTS    += -Wno-strict-prototypes
+rts/sm/Storage_CC_OPTS  += -Wno-strict-prototypes
 # ffi.h triggers undefined macro warnings on PowerPC, disable those:
 # this matches substrings of powerpc64le, including "powerpc" and "powerpc64"
 ifneq "$(findstring $(TargetArch_CPP), powerpc64le)" ""