[chromecast] Remove unused/unimplemented GetInstance

Code cleanup as per TODO from eons ago. This method is not used, and
is no longer even implemented.

Bug: none
Test: manual and unit tests
Change-Id: I7caf825f3be486e2bf88294f2e0b6eb92baf712f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032003Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737324}

chromecast/browser/tts/tts_platform.h

@@ -18,11 +18,6 @@ class TtsPlatformImpl {
 
   virtual ~TtsPlatformImpl() {}
 
-  // TODO(rdaum): Delete declaration after downstream implementation is using
-  // SetPlatformImpl. (This is no longer used and the platform used is selected
-  // by SetPlatformImpl on TtsController instead)
-  static TtsPlatformImpl* GetInstance();
-
   // Returns true if this platform implementation is supported and available.
   virtual bool PlatformImplAvailable() = 0;