UBI: remove UBI_IO_DEBUG macro

This additional little macro is used to print a bit more messages
while scanning the media. However, we have the 'dbg_bld()' macro
for this, so we better us 'dbg_bld()' and kill UBI_IO_DEBUG. This
simplifies the code a tiny bit.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>