STYLE: remove unused backwards compatibility code

This snippet of code was used during the initial porting of code
from slicer3 to slicer4 but has since been replaced with direct access
to the corresponding items via the regular layoutManager API.

This code also had the unwanted restriction that all Slice View
layout names had to be valid python identifiers (no hyphens or
spaces) which led to spurious syntax error messages for some use
cases.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23326 3bd1e089-480b-0410-8dfb-8563597acbee