ksmbd: remove unused is_char_allowed function

clang with W=1 reports
fs/ksmbd/unicode.c:122:19: error: unused function
  'is_char_allowed' [-Werror,-Wunused-function]
static inline int is_char_allowed(char *ch)
                  ^
This function is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>