mem-ruby: Remove maskHighOrderBits

mem-ruby: Remove maskHighOrderBits

Function was not being used. If needed, src/base/bitfield.hh's
mbits can be used instead:

maskHighOrderBits(addr, pos) == mbits<Addr>(addr, 64-pos, 0)

Change-Id: I3abd041f8d256ec157ba7502182d8588721c2a05
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21082
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>