[LibOS] Remove CFLAGS-libos from LibOS/shim/test/benchmark/Makefile

It's already defined in LibOS/shim/test/Makefile.Test.

LibOS/shim/test/benchmark/Makefile

@@ -8,8 +8,6 @@ include ../../../../Makefile.configs
 include ../../../../Makefile.rules
 include ../Makefile.Test
 
-CFLAGS-libos = -L../../../glibc-build/libos -I../../include
-
 %.manifest: %.manifest.template
 	rm -rf $@
 	cp $@.template $@