Remove -Wunused-const-variable warning

-Wunused-const-variable=1 is already included by -Wunused-variable,
which is part of -Wall. -Wunused-const-variable=2, which is what we have
been using as part of W=1, warns for unused static const variables in
headers, which will likely produce a lot of false positives that will
take a large effort to fix.

Additionally, some of these issues may be caused by different builds of
TF-A where some features are used in some builds and ignored in others.

Change-Id: Ifa0b16a75344cc1f6240e8d5745005f8f2046d34
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>

Makefile

@@ -240,7 +240,6 @@ WARNING1 += -Wmissing-declarations
 WARNING1 += -Wmissing-format-attribute
 WARNING1 += -Wmissing-prototypes
 WARNING1 += -Wold-style-definition
-WARNING1 += -Wunused-const-variable
 
 # Level 2
 WARNING2 := -Waggregate-return