rm deprecated method in BackendCompilationUtilities

Remove a deprecated method inside BackendCompilationUtilities that
relies on FIRRTL Driver Compatibility phases.  This is long overdue for
removal and will need to get ripped out for Chisel 5 as this removes
the (also deprecated) SFC phases that back this.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>