D3D11: Remove unused field in InputLayoutCache. The mCounter field became exposed as unused after it was initialied in the constructor initializer list. Previously this was hidden because it was initialized in the body of the constructor. BUG=angleproject:1327 Change-Id: I18d9aa8a377d234f4c161ace845edfb9fc7f226c Reviewed-on: https://chromium-review.googlesource.com/331385 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>