[Remoting iOS] Remove applyColorSchemes

[MDCAlertColorThemer applyColorScheme:] has been deprecated. We no
longer use MDCAlert after we removed the data usage warning dialog, so
it's easier to just remove the applyColorSchemes method altogether.

Bug: 930714
Change-Id: Ifa9742b901ffcb0af739a5f79a2440792ad911e1
Reviewed-on: https://chromium-review.googlesource.com/c/1469404
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631834}
3 files changed