[Documentation] Remove some z-Specific info from Power Install Doc

There are some copy-paste issues with the Installing on Power section. Here is the 4.12 link: https://access.redhat.com/documentation/en-us/openshift_container_platform/4.12/html/installing/installing-on-ibm-power#preparing-to-install-on-ibm-power

Remove:
1) the note about distributing the nodes over z/VM instances
2) any mention of z-specific terms, e.g. IFLs

1.
2.
3.