releasetools: Kill the use of os.getenv('AVBTOOL').

OTA tools should pick up the avbtool, as listed in dict['avb_avbtool'],
from the current PATH (plus bin/ under the dir specified via `--path`),
the same way as handling all other host tools.

Test: `m dist`
Change-Id: I3eb4d2c61979b03d9c23b2403d9a38cf052d87ea