s390/als: Remove obsolete comment

The bss section of the decompressor is part of the compressed kernel image
since commit 980d5f9a ("s390/boot: enable .bss section for compressed
kernel").

Remove a now incorrect comment that states that the bss section must not be
accessed.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>