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Remove gmock from managed_network_configuration_handler_unittest.cc part 5.

Now, all tests are migrated to non-gmock test cases.
Clean up unnecessary code.

BUG=234463
TEST=Ran bots.

Change-Id: I71861d8b70544764c6b149574730f1abda007250
Reviewed-on: https://chromium-review.googlesource.com/819190Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523371}