s390: remove unused _ehead symbol

Since startup code now reserves memory ranges [0, PARMAREA_END] and
[_stext, <end of kernel>] _ehead symbol is not used and could be
cleaned up.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>