Remove GKI_SIGNING args from recovery & debug boot image build rules

These options are for certifying official GKI release, so they should
never be added to build command of recovery.img and boot-debug.img.

Also define gki_signing_signature_args in misc_info.txt iff.
BOARD_GKI_SIGNING_KEY_PATH is defined, so there's one less conditional.
We need to define it even if BOARD_GKI_SIGNING_SIGNATURE_ARGS is empty,
so that sign_target_files_apks.py can modify the option during image
signing.

Bug: 210367929
Test: Presubmit
Change-Id: I9546fcda796586d97af244955e15c1b8dc9f6d92