Remove SigninManager's leftover includes from EPK private API

There is no piece of code currently using the SigninManager so we can
safely remove them.

Bug: 920207
Change-Id: If1235d83211c15c02f0fbafc5ec9b6a02125565e
Reviewed-on: https://chromium-review.googlesource.com/c/1407700
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Commit-Queue: Sergio Villar <svillar@igalia.com>
Cr-Commit-Position: refs/heads/master@{#624800}
1 file changed