Remove three board_avb_* args from META/misc_info.txt.

board_avb_algorithm and board_avb_key_path are overlapping with
avb_signing_args. In core/Makefile, only avb_signing_args (i.e.
INTERNAL_AVB_SIGNING_ARGS) will be used in the AVB-signing command. It
covers the contents in board_avb_{algorithm,key_path}. We should do the
same thing in tools/releasetools to avoid potential inconsistency.

This CL cleans up the logic in tools/releasetools, by always using
avb_signing_args. This also allows easier signing key replacement (so we
can replace the key/algorithm/signer in 'avb_signing_args').

board_avb_system_add_hashtree_footer_args is unused in releasetools
script, and the same information has been covered by
system_avb_add_hashtree_footer_args. This CL removes this arg as well.

Test: `m dist`. Then a) check the removed three args no longer exist in
      META/misc_info.txt; b) check that rebuilding images with
      add_img_to_target_files.py uses the same parameters.
Change-Id: I7db890b5c942de5b6868d8d1ebf937586d4729c0