[signin] Delete GetSigninErrorController from profile_state

This function was just a straight wrapper around the
SigninErrorControllerFactory but is not defined on ChromeOS. This does
not seem very valuable.

Moreover, it was only used from AvatarButtonErrorController.
And since AvatarButtonErrorController will be needed on ChromeOS, there is
no point in keeping this wrapper.

Bug: 1041472
Change-Id: If6f7e84dc9250e3d542206022a20620235eb885a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093597
Reviewed-by: Monica Basta <msalama@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748688}