Remove mbrtowc_invalid_utf8.cpp

This was used in CMake to detect invalid mbrtowc implementations. The only known
case was on SnowLeopard, which is no longer supported. Remove this file.

cmake/checks/mbrtowc_invalid_utf8.cpp

@@ -1,18 +0,0 @@
-#include <cstdio>
-#include <cstdlib>
-#include <cstring>
-#include <cwchar>
-
-// Check whether the runtime mbrtowc implementation attempts to encode
-// invalid UTF-8 values.
-
-int main() {
-    // TODO: I'm not sure how to enforce a UTF-8 locale without overriding the language
-    char sample[] = "hello world";
-    sample[0] |= 0xF8;
-    wchar_t wsample[100] {};
-    std::mbstate_t state = std::mbstate_t();
-    int res = std::mbrtowc(wsample, sample, strlen(sample), &state);
-
-    return res < 0 ? 0 : 1;
-}