Fix require spec for pathname builtin

Remove redundant rb_provide("pathname") since pathname.so and
pathname.rb already cover all require patterns. Add uniq! to the
require spec to handle pathname appearing twice in $LOADED_FEATURES
(once for .so, once for .rb).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

pathname.c

@@ -121,7 +121,6 @@ InitVM_pathname(void)
 
     rb_provide("pathname.so");
     rb_provide("pathname.rb");
-    rb_provide("pathname");
 }
 
 void

spec/ruby/core/kernel/require_spec.rb

@@ -36,6 +36,7 @@
       features -= %w[encdb transdb windows_1252 windows_31]
     end
     features.reject! { |feature| feature.end_with?('-fake') }
+    features.uniq!
 
     features.sort.should == provided.sort