Remove attempt to link xml_ruby in extconf.rb.

Proposing that extconf.rb not link xml_ruby at all, as it doesn't seem to use any of the xml_ruby libraries when compiling. Instead rely on existing checks to ensure that libxml-ruby is installed, the existing checks being: 1) the [search for libxml-ruby's installation path in extconf.rb](https://github.com/xml4r/libxslt-ruby/blob/090f0c548031b87dbc5657739a4da4a97977924f/ext/libxslt/extconf.rb#L114), and 2) the [.gemspec dependency upon libxml-ruby](https://github.com/xml4r/libxslt-ruby/blob/090f0c548031b87dbc5657739a4da4a97977924f/libxslt-ruby.gemspec#L19).

(We'd also like to remove this section of the extconf.rb because it prevents libxml-ruby from building on Rubinius 1.3.3, due to the linker throwing ["undefined reference"](https://gist.github.com/sshao/e3920428fd7c9475bdb7) errors at this stage. Rubinius does not generate any library/shared object files to link against.)

This is tested on Debian Wheezy (libxml 2.8.0, libxml 1.1.26) with both ruby MRI 1.8.7-p358 and rubinius 1.3.3.