Remove aura::Env from WMHelperCastShell

Somehow I've overseen this, but r660484 removed WMHelper::env()
method, so this class also doesn't need it anymore. Removing.

Bug: 958467
Test: none
Change-Id: I9a1c38b65e33c25f086cd81ac2cb0fa079690db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617087
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661826}
3 files changed