back.verilog: remove $verilog_initial_trigger after proc_prune.

back.verilog: remove $verilog_initial_trigger after proc_prune.

$verilog_initial_trigger was introduced to work around Verilog
simulation semantics issues with `always @*` statements that only
have constants on RHS and in conditions. Unfortunately, it breaks
Verilator. Since the combination of proc_prune and proc_clean passes
eliminates all such statements, it can be simply removed when both
of these passes are available, currently on Yosys master. After
Yosys 0.10 is released, we can get rid of $verilog_initial_trigger
entirely.