No need to JAR-sign OTA update packages.

This removes the logic for JAR signing from -w (whole-file signing)
mode. This mode is designed specifically for OTA update packages. When
such packages are verified, their JAR signatures are ignored. Thus,
there is no need to JAR-sign in -w mode.

For context, OTA update packages are protected by a special signature
residing in the ZIP End of Central Directory record (at the very end
of the file). This is the signature verified when update packages are
being applied to Android.

Change-Id: Ia852a11ed6774ce746087cdd7f028b191ef6bc8b