Remove unused signin_manager getter from account_investigator_unittest.cc

This is an oneliner removal follow up of crrev.com/c/1351152.

TBR=msarda@chromium.org
BUG=909715

Change-Id: I8bc16bfb73348c23425351761695cca2258342c6
Reviewed-on: https://chromium-review.googlesource.com/c/1396237Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#619966}

components/signin/core/browser/account_investigator_unittest.cc

@@ -62,7 +62,6 @@ class AccountInvestigatorTest : public testing::Test {
 
   ~AccountInvestigatorTest() override { investigator_.Shutdown(); }
 
-  FakeSigninManager* signin_manager() { return &signin_manager_; }
   identity::IdentityTestEnvironment* identity_test_env() {
     return &identity_test_env_;
   }