ExtJS: Use provided copy instead of overwriting with libsjs-extjs.

libsjs-extjs in Debian squeeze (as of writing) is at 3.0.3,
which is incompatible with JSDuck (it currently assumes 4.1).

The fetch from Gemwatch already includes everything it needs
in "template-min/". Nothing needs to be replaced or pre-processed.

Change-Id: Ib3e813d57780c7e89b67bd479860558a51104758