Remove no longer required strnlen wrapper

It was used for MinGW & OS X < 10.7. Minimal OS X version was bumped to
10.7 and MinGW support was recently dropped (see
1eaa64c49c).

components/esm/esmcommon.hpp

@@ -5,7 +5,7 @@
 #include <cstring>
 
 #include <stdint.h>
-#include <libs/platform/string.h>
+#include <string.h>
 
 namespace ESM
 {

components/esm/esmreader.hpp

@@ -2,7 +2,7 @@
 #define OPENMW_ESM_READER_H
 
 #include <stdint.h>
-#include <libs/platform/string.h>
+#include <string.h>
 #include <cassert>
 #include <vector>
 #include <sstream>

libs/platform/string.h

@@ -1,34 +0,0 @@
-// Wrapper for string.h on Mac and MinGW
-#ifndef _STRING_WRAPPER_H
-#define _STRING_WRAPPER_H
-
-#ifdef __APPLE__
-#include <Availability.h>
-#endif
-
-#include <string.h>
-#if (defined(__APPLE__) && __MAC_OS_X_VERSION_MIN_REQUIRED < 1070) || defined(__MINGW32__)
-// need our own implementation of strnlen
-#ifdef __MINGW32__
-static size_t strnlen(const char *s, size_t n)
-{
-    const char *p = (const char *)memchr(s, 0, n);
-    return(p ? p-s : n);
-}
-#elif (defined(__APPLE__) && __MAC_OS_X_VERSION_MIN_REQUIRED < 1070)
-static size_t mw_strnlen(const char *s, size_t n)
-{
-    if (strnlen != NULL) {
-        return strnlen(s, n);
-    }
-    else {
-        const char *p = (const char *)memchr(s, 0, n);
-        return(p ? p-s : n);
-    }
-}
-#define strnlen mw_strnlen
-#endif
-
-#endif
-
-#endif