write_verilog: do not print (*init*) attributes on regs.

write_verilog: do not print (*init*) attributes on regs.

If an init value is emitted for a reg, an (*init*) attribute is never
necessary, since it is exactly equivalent. On the other hand, some
tools that consume Verilog (ISE, Vivado, Quartus) complain about
(*init*) attributes because their interpretation differs from Yosys.

All (*init*) attributes that would not become reg init values anyway
are emitted as before.