arc: remove redundant UTS_MACHINE define in arch/arc/Makefile

The top-level Makefile sets the default of UTS_MACHINE to $(ARCH).

If ARCH and UTS_MACHINE match, arch/$(ARCH)/Makefile need not specify
UTS_MACHINE explicitly.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>