[FIRTOOL] -verilog flag should not include firrtl_black_box_resource_files.f

Nobody has claimed this yet.

As discussed in https://github.com/llvm/circt/issues/2254 , `firtool -verilog` will append the contents of the `firrtl_black_box_resource_files.f` file into the output verilog file. This results in non-compliant verilog. Since these BlackBox modules are included in the output verilog the `firrtl_black_box_resource_files.f` doesn't appear to be useful anyways.

`-split-verilog` does not have this issue.

Below is an example

```verilog
//previous valid verilog
endmodule
// /home/lsteveol/projects/wlink_test/wav-wlink-hw/output-chirrtl/Wlink.fir:33116:13

// ----- 8< ----- FILE "firrtl_black_box_resource_files.f" ----- 8< -----

./wav_latch_model.sv
./wlink_WavFIFOMem.v
./wlink_WavFIFOPtrLogic.v
./wlink_WavReplayFIFOPtrLogic.v
./wlink_wlink_axi_arFC_a2l_93x8.v
./wlink_wlink_axi_arFC_l2a_93x8.v
./wlink_wlink_axi_awFC_a2l_93x8.v
./wlink_wlink_axi_awFC_l2a_93x8.v
./wlink_wlink_axi_bFC_a2l_6x8.v
./wlink_wlink_axi_bFC_l2a_6x8.v
./wlink_wlink_axi_rFC_a2l_39x32.v
./wlink_wlink_axi_rFC_l2a_39x32.v
./wlink_wlink_axi_wFC_a2l_37x32.v
./wlink_wlink_axi_wFC_l2a_37x32.v // /home/lsteveol/projects/wlink_test/wav-wlink-hw/output-chirrtl/Wlink.fir:2:1

EOF
```

Start at the firtool handling for the -verilog and -split-verilog output modes, then compare how firrtl_black_box_resource_files.f is incorporated. Reproduce the example behavior and confirm that -verilog emits compliant Verilog without appending the resource-file contents, while -split-verilog remains unaffected.

Written by the indexing model from the issue text.