Remove unused MRuby::Build#list_install_excludes method

This method was added by commit 0ac755dbc8 (#6407).
It was added during the initial conception, but was ultimately no longer needed.

lib/mruby/build.rb

@@ -542,14 +542,6 @@ EOS
       @install_prefix = dir&.to_s
     end
 
-    def list_install_excludes
-      install_excludes.flatten.flat_map { |e|
-        e.kind_of?(Proc) ? e.call(self) : e
-      }.map { |e|
-        File.join(build_dir, e)
-      }
-    end
-
     protected
 
     attr_writer :presym