Fixed latency spike

A spike in the startup time of DisplayManagerService was observeed
because of a recent change where we are attempting read values from
DeviceConfig, which if missing fallsback to DisplayDeviceConfig. This
extra read attempt is unneeded because once the DisplayManagerService is
setup, we explicitly make a start call to DisplayModeDirector which
takes care of reloading values from DeviceConfig if present.

Bug: 254354520
Test: Manual
Change-Id: Iae7066b8c732d9b39e3fb76200ff24cb5aeb3d73