related to R4813, removes redundant notice-disabling lines

This is related to [R4813](http://dev.kohanaframework.org/issues/4813) and #444

It is already fixed in Kohana 3.3 by @rjd22 in #363 and kohana/kohana#12

I just removed redundant notice-disabling lines because `mb_convert_encoding`, unlike `iconv`, does not emit `E_NOTICE`.

Please review and merge :) Thanks!