s390/kdump: Use 64 bit mode for 0x10000 entry point

The 0x10000 entry point can be called in z/Arch architecture and 64 bit
addressing mode. Therefore this patch removes the unnecessary 31 bit
switch code from the kdump startup function.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>