Remove unnecessary `require 'sass'` lines

This lines are a historical artifact of a time before we used the
Visitor pattern for the `Linter` class. Since `lib/scss_lint.rb`
`require`s the `sass` library in order to monkey patch its parse tree,
we don't need to duplicate those require statements.

Change-Id: I8c3a569b4ef7ca40e365a234b341312e7f5c3687
Reviewed-on: https://gerrit.causes.com/27806
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>