remove init_shell

This domain was originally intended to be a place to hold rules for
all init.*.rc shell scripts. However, it's now recommended that every
init service have it's own SELinux domain, and the use of init_shell
is to be avoided.

Delete init_shell. No policy is using it anymore, and it's causing
confusion for people implementing device specific SELinux policy.

Bug: 18062250
Change-Id: I7c90851784b233443642ea69722f3281fd457621