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 200be338e313e23543f98c36a5127b3e22c6f5ab)

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>
diff --git a/update_engine.gyp b/update_engine.gyp
index c80c988..d14db2f 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -23,12 +23,8 @@
       # The -DUSE_* flags are passed from platform2.py. We use sane defaults
       # here when these USE flags are not defined. You can set the default value
       # for the USE flag in the ebuild.
-      'USE_binder%': '0',
       'USE_dbus%': '1',
       'USE_hwid_override%': '0',
-      'USE_libcros%': '1',
-      'USE_mtd%': '0',
-      'USE_power_management%': '0',
     },
     'cflags': [
       '-g',