Remove support for Phar archives

Bundling WP-CLI into a Phar archive was an interesting experiment, but it's become unnecessary since the introduction of the installer script: wp-cli/wp-cli.github.com#5

* can't install bash completions
* can't easily hack on WP-CLI files
* can't contain optional packages: wp-cli/wp-cli#437

We'll keep http://wp-cli.org/packages/phar/wp-cli.phar for a while, but it won't be updated anymore.