Remove extraneous re-addition of package list to package list.

Simple edit. To match the pattern used below in the Red Hat portion of the case statement. `+=` already takes care of including the original list of Debian packages.

The repetition has been safe to repeat—thanks Chef package resource idempotency—but should probably be removed to save Chef trying to reinstall the original list of packages again.

Caught while walking through the code with @LuqiPan