Remove dead MapperBusyException
Remove dead MapperBusyException

This exception class was not removed when its usage disappeared in
d43087f294949583507b5520c8f0b2c359296aa1.

Change-Id: I937f78ac37018ad59e8cb5fc65819497126f02d9
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/phosphor-objmgr,openbmc/phosphor-objmgr