Remove the ARCH build configuration from the Makefile

The ARCH variable, which defaults to 'aarch64', gives the wrong
impression that the Trusted Firmware can be built for other
architectures. This patch removes it. This doesn't have any
consequence on the rest of the build system because the variable
was unused.

Change-Id: I97130f11f7733a3cbdfc89989587f2ebecaf3294

Makefile

@@ -42,8 +42,6 @@ VERSION_MINOR		:= 1
 V			:= 0
 # Debug build
 DEBUG			:= 0
-# Build architecture
-ARCH 			:= aarch64
 # Build platform
 DEFAULT_PLAT		:= fvp
 PLAT			:= ${DEFAULT_PLAT}