patman: Don't convert input data to unicode

The communication filter reads data in blocks and converts each block to
unicode (if necessary) one at a time. In the unlikely event that a unicode
character in the input spans a block this will not work. We get an error
like:

UnicodeDecodeError: 'utf8' codec can't decode bytes in position 1022-1023:
   unexpected end of data

There is no need to change the input to unicode, so the easiest fix is to
drop this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>