parisc: remove unused arch/parisc/boot/install.sh and its phony target

Parisc has two similar installation scripts:

  arch/parisc/install.sh
  arch/parisc/boot/install.sh

The latter is never used because 'make ARCH=parisc install' always
invokes the 'install' target in arch/parisc/Makefile.

The target in arch/parisc/boot/Makefile is not used either.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>