Platforms/RaspberryPi: Remove DebugShowUEFIExit

The "Verbose ExitBootServices" feature was originally added to the RPi
as part of early OS enablement to show that the OS boot loader did
actually call ExitBootServices (back when OS boot used to crash shortly
after that). This is no longer needed, and should be removed as part of
cleaning the RPi PlatformBootManager to be more in-line with the ArmPkg
version.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Pete Batard <pete@akeo.ie>
Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>