mem-ruby: Remove function that maps responses to a DMA engine

mem-ruby: Remove function that maps responses to a DMA engine

The function map_Address_to_DMA was used to route responses to the
first (and assumed to be the only) DMA engine in the system. This
function is now unused as protocols handle responses and route them to
the right DMA engine.

Change-Id: I2fba913cf2f12321d1a1e38e7ee85bdf26b8a47a
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/7162
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>