Remove obsolete option --enable-mathn

The code has been removed already, and the latest mathn is refactored
to use refinements.

Author: nobu

configure.ac

@@ -1804,10 +1804,6 @@ RUBY_DECL_ATTRIBUTE([__nonnull__(n)], [RUBY_FUNC_NONNULL(n,x)], [rb_cv_func_nonn
 
 RUBY_APPEND_OPTION(XCFLAGS, -DRUBY_EXPORT)
 
-AC_ARG_ENABLE(mathn,
-    AS_HELP_STRING([--enable-mathn], [enable canonicalization for mathn]),
-    [AC_MSG_ERROR([mathn support has been dropped])])
-
 AC_CACHE_CHECK(for function name string predefined identifier,
     rb_cv_function_name_string,
     [AS_CASE(["$target_os"],[openbsd*],[