udf: Drop VARCONV support

UDF was supporting a strange mode where the media was containing 7
blocks of unknown data for every 32 blocks of the filesystem. I have yet
to see the media that would need such conversion (maybe it comes from
packet writing times) and the conversions have been inconsistent in the
code. In particular any write will write to a wrong block and corrupt
the media. This is an indication and no user actually needs this so
let's just drop the support instead of trying to fix it.

Signed-off-by: Jan Kara <jack@suse.cz>