levels: MAP15/MAP28 2013-08-30 updates from Z0k.

Change comments:

Map15:

-Removed an impassible line in Red key area, now cacodemons can reach
players without trouble.

-Replaced an Imp for a Cacodemon

-Removed block sound lines in the red key area where 2 shotgunners can
be found near of the lift.

-Added "block Monster" lines for the imps that are near of the
chaingunner and shotgunners. so imps cannot be killed by them.

Map28

-Removed 4 chaingunners on the yellow key swicht, now the enemies that
will appear there would be defined by the skill level:

Easy: Zombieman
Normal: Shotgunner
Hard: Shotgunner

-Removed 2 hell knights on the yellow key area, now they are replaced
by imps.

-Removed the floor damaged on the blue key are where players find the
swicht where the rocket launcher is found.

-Removed the revenants on the blue key area near of the bloodfalls.
now they are replaced by chaingunners on all skills. this change was
made due revenants were to hard to kill in that place and also a waste
of ammo, now chaingunners provide a bit of challenge while they can be
killed easily.

Those are the final revisions for the maps. and i dont going to update
them until more players tells if the maps are still harder for them

Signed-off-by: Simon Howard <fraggle@gmail.com>