Remove primary_user_ from FakeUserManager.

UserManagerBase already has primary_user_, and it is used
in the base class.
Currently, FakeUserManager has yet another primary_user_ field,
so the base class's functions are not properly working.

This CL removes the FakeUserManager::primary_user_, so the
remaining code will use UserManagerBase::primary_user_.

BUG=1325210
TEST=Run tryjob.

Change-Id: Iaa1afded80cae6e65725bb22e8ef3a1472efcfc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4197761
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1098382}