Makefile: DEBUG should be 1 or 0

This is already set in the COPTS so remove this conditional setting of
the DEBUG macro.

Change-Id: I4ec8d7fa527ad4943d80a53c6418b81f7dc5446c
diff --git a/Makefile b/Makefile
index 7a0b08f..fea072d 100644
--- a/Makefile
+++ b/Makefile
@@ -70,10 +70,6 @@
   COPTS += -DPL011_BASE=$(PL011_BASE)
 endif
 
-ifeq ($(DEBUG),1)
-  COPTS += -DDEBUG
-endif
-
 define build_c
   TGT := $(patsubst %.c,%.o,$(OUT)/$(patsubst src/%,%,$(1)))
   GLOBAL_OBJS += $$(TGT)