* doc/emms.texinfo: Remove reference to Last.fm support.

Last.fm support has failed to track the API and licensing changes over
at Last.fm. While I wouldn't remove the code just yet, we should
remove it from the manual so that people don't get the impression that
it is actually working.
* lisp/emms-setup.el: Remove Last.fm extension from setup.

This code currently doesn't work, so we don't need it loaded in the
setup.
Fix compilation warnings.
* lisp/emms-tag-editor.el: Silence the compiler by explicitly passing `filename'.