[S390] 3215/3270 console: remove wrong comment

When converting these two drivers from bootmem to slab allocations I
forgot to remove two comments which state that this wouldn't be
possible.
So remove them now since this caused confusion already.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>