Remove contra productive _string_check

_string_check is rather harmful when using the new filtering. It creates
non-unicode strings out of valid strings.
The new unicode validation code covers the boundaries much better than
_string_check therefore we remove it.

Change-Id: Id01d8dc1374ae33435c5aefaae559ab8aa0ff062
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>