Remove redundant variables from gyp file

Remove a bunch of variables that are already defined in the common.gypi file in
platform2.  Since this file is automatically included in all platform builds
redefining them here doesn't actually do anything.

(cherry picked from commit 200be338)

BUG=none
TEST='FEATURES=test emerge-$BOARD update_engine' still works

Change-Id: Ibbb2dde6f1285d95d7d6bc31187caea3e3cad43c
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/457870


Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>