Remove left over bits of the MARKSWEEP option

Ref #11811

Author: yuyichao

Make.inc

@@ -267,7 +267,6 @@ fPIC := -fPIC
 EXE :=
 endif
 
-JULIAGC := MARKSWEEP
 JULIACODEGEN := LLVM
 
 # flag for disabling assertions

src/Makefile

@@ -29,11 +29,7 @@ SRCS := \
 	jltypes gf typemap ast builtins module interpreter \
 	alloc dlload sys init task array dump toplevel jl_uv jlapi signal-handling \
 	simplevector APInt-C runtime_intrinsics runtime_ccall \
-	threadgroup threading stackwalk
-
-ifeq ($(JULIAGC),MARKSWEEP)
-SRCS += gc
-endif
+	threadgroup threading stackwalk gc
 
 ifeq ($(JULIACODEGEN),LLVM)
 SRCS += codegen disasm debuginfo llvm-simdloop llvm-gcroot