extconf.rb: remove dir_config("kerberos")

The dir_config was introduced by Ruby trunk r4181. Since support for
Kerberos cipher suites has been removed in OpenSSL 1.1.0, it is no
longer necessary.

Although ruby/openssl did not directly depend on the MIT Kerberos
library, it was an optional transitive dependency. Unless it was
disabled by a compile-time option, the OpenSSL headers would try to
include <krb5.h>.

Author: rhenium

ext/openssl/extconf.rb

@@ -34,8 +34,6 @@
     "library directories."
 end
 
-dir_config("kerberos")
-
 Logging::message "=== OpenSSL for Ruby configurator ===\n"
 
 $defs.push("-D""OPENSSL_SUPPRESS_DEPRECATED")