Remove unused functions `bzbug` and `is_bzbug`

Also remove now-unnecessary imports.
Move method to different module. Fix imports.

Move method `skip_if_bz_bug_open` from module `robottelo.common.decorators` to
`robottelo.common.helpers`. Do this b/c the method is not a decorator. Fix
imports to accomodate this change.
Refactor method `skip_if_bz_bug_open`

Make method include only one return statement, not two.