game_join_processing.php: use SmrPlayer::getHome
game_join_processing.php: use SmrPlayer::getHome

The code to determine the starting sector duplicates the
code in `SmrPlayer::getHome`. Remove this code in favor
of using the class method.

hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr,smrealms/smr