Remove c2 Makefile target

I don't believe this backend exists any more.

Makefile

@@ -248,10 +248,6 @@ generated_definitions/c/riscv_model_$(ARCH).c: $(SAIL_SRCS) model/main.sail Make
 	mkdir -p generated_definitions/c
 	$(SAIL) $(SAIL_FLAGS) $(c_preserve_fns) -O -Oconstant_fold -memo_z3 -c -c_include riscv_prelude.h -c_include riscv_platform.h -c_no_main $(SAIL_SRCS) model/main.sail -o $(basename $@)
 
-generated_definitions/c2/riscv_model_$(ARCH).c: $(SAIL_SRCS) model/main.sail Makefile
-	mkdir -p generated_definitions/c2
-	$(SAIL) $(SAIL_FLAGS) -no_warn -memo_z3 -config c_emulator/config.json -c2 $(SAIL_SRCS) -o $(basename $@)
-
 $(SOFTFLOAT_LIBS):
 	$(MAKE) SPECIALIZE_TYPE=$(SOFTFLOAT_SPECIALIZE_TYPE) -C $(SOFTFLOAT_LIBDIR)