Fix emerald hunt stages not giving rewards upon completion

This MR fixes a bug where completing an emerald hunt stage doesn't reward the player with bonuses, emblems, or unlockables associated with the stage. This is due to a change in 2.2.14 where the PF_FINISHED flag must be applied to a player for them to get rewards from completing a stage.

This MR fixes this by using P_DoPlayerExit with the finishedflag argument set to true, in place of manually setting the player's exiting variable.

Special thanks to InsaneEnergy on the SRB2 Discord server for bringing attention to this bug!