parisc: Always use the self-extracting kernel feature

This patch drops the CONFIG_PARISC_SELF_EXTRACT option.

The palo boot loader is able to decompress a kernel which was compressed
with gzip. That possibility was useful when the Linux kernel
self-extracting feature wasn't implemented yet.

Beside the fact that the self-extracting feature offers much better
compression rates, we do support self-extracting kernels already since
kernel v4.14, so now it's really time to get rid of that old option and
always use the self-extractor.

Signed-off-by: Helge Deller <deller@gmx.de>