build: assume snprintf() in mprintf, drop feature check

- it was already required for `curl_*printf()` float/double support.
- some curl tests always fail without it.
- it was already assumed to be present to build test servers.
  Source code did not check for `HAVE_SNPRINTF` detection variable.
- it was already required to build examples.

Windows builds stopped using this detection and the function via earlier
commits.

Follow-up to 64f28b8f88 #20765
Follow-up to 935b1bd454 #9570 #9569

Closes #20763

lib/mprintf.c

@@ -678,14 +678,11 @@ static bool out_double(void *userp,
 #endif
 #ifdef _WIN32
   curlx_win32_snprintf(work, BUFFSIZE, fmt, dnum);
-#elif defined(HAVE_SNPRINTF)
+#else
   /* !checksrc! disable BANNEDFUNC 1 */
   /* !checksrc! disable LONGLINE */
   /* NOLINTNEXTLINE(clang-analyzer-security.insecureAPI.DeprecatedOrUnsafeBufferHandling) */
   snprintf(work, BUFFSIZE, fmt, dnum);
-#else
-  /* float and double outputs do not work without snprintf support */
-  work[0] = 0;
 #endif
 #ifdef CURL_HAVE_DIAG
 #pragma GCC diagnostic pop