fix disable plugin mechanism revert FS#1585

Ignore-this: bd7e66b737e8847373e071f0b23d96d7

The mechanism for disabling plugins was not reverted completely: there were
still references to the plugin.disabled directories in the
Doku_Plugin_Controller class, this patch fixes this.

darcs-hash:20090203194307-8a46b-4bb1c41e530edc9fde06ca61898a9a465d5b6d98.gz