dev, arm: remove the RealViewEB platform

dev, arm: remove the RealViewEB platform

This is an old platform, and we haven't had official Linux kernel configs
for it in a while, so we've decided to deprecate it.

Furthermore, trying to use it fails with:

object 'RealViewEB' has no attribute 'pci_host'

and the last commit in the class happened two years ago, which indicates
that no one has been using it.

Change-Id: Icc674b00b152eb3246e05141dbaf2624cc720f21
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/12471
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>