LibC: Remove the mbstate_reset helper

A zero-initialized mbstate_t struct has to be a valid initial state, so
we can just zero-initialize it whenever we need to reset.

Having a helper function for resetting the struct might imply that you
can add additional setup operations afterwards, which is not the case.

Userland/Libraries/LibC/wchar.h

@@ -18,6 +18,7 @@ __BEGIN_DECLS
 typedef __WINT_TYPE__ wint_t;
 typedef unsigned long int wctype_t;
 
+// A zero-initialized mbstate_t struct must be a valid initial state.
 typedef struct {
     unsigned char bytes[4];
 } mbstate_t;