s13n: Remove SigninManager-related includes from identity_apitest.cc

This CL cleans up the unnecessary includes related to SigninManager.

Bug: 920210
Change-Id: I67b9505ad35270ca50356dade8e02fb478557000
Reviewed-on: https://chromium-review.googlesource.com/c/1414730
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#623173}
1 file changed