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

configure.ac

@@ -4198,7 +4198,6 @@ if test "$curl_cv_native_windows" != "yes"; then
     if_nametoindex \
     realpath \
     sched_yield \
-    snprintf \
   ])
   CURL_CHECK_FUNC_INET_NTOP
   CURL_CHECK_FUNC_INET_PTON