Remove special http namespace, libapreq2 dependency which complicates builds

[lrowe]

The special http namespace is non standard. It requires a dependency on libapreq2 which is tricky to build and the plugin system is otherwise unused.

• Remove libapreq2 dependency from build system. lrowe/mod_transform@cc341d8
• Remove all of the special http namespace functions, removing libapreq2 usage.
  lrowe/mod_transform@2c8e51a
• Remove the now unused plugin system
  lrowe/mod_transform@04e6014

To support the use case of the http namespace it would be better to support passing parameters (accessed by xsl:param). There is an old patch in the bug tracker supporting that (http://issues.outoforder.cc/view.php?id=74) but it should really support optional quoting as the nginx xslt module does with xsl_param/xsl_string_param.