mtd: remove MTDDEBUG() and CONFIG_MTD_DEBUG

All users of this macro have been converted.  Remove MTDDEBUG and
related CONFIG options.

ubifs_dbg_msg_key() is kept.  It is silent unless DEBUG is defined.

I am not touching scripts/config_whitelist.txt.  The deprecated options
will be dropped by the next resync.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>