releasetools: Remove the deprecated flag --board_config.

It has been long deprecated since commit
fdd8e69c (Donut).

Test: `m dist` with aosp_marlin-userdebug and aosp_angler-userdebug.
Test: No user of "--board_config" in code search.
Change-Id: Ic0336c22cac32de5de88161748d09b579f892e99