string.c: remove str_mod_check

Clang 3.4 emits '-Wunused-function' - and it's really unused!
But according to the description this seems to be a bug:

>Warn whenever a static function is declared but not defined or
>a non-inline static function is unused. This warning is enabled by
-Wall.

Source: <http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html>
Merge pull request #1650 from cremno/remove-str_mod_check

string.c: remove str_mod_check