Remove default_nettype statements at the end of the RTL

[kareefardi]

Surfaced by The-OpenROAD-Project/OpenLane#2083. Yosys would use the default nettype wire when compiling the user project. Although this might be an ambiguity in yosys behavior, I don't see the need of adding these lines. Having said that, there presence in the RTL seems to be intentional, I added @marwaneltoukhy and @M0stafaRady as reviewers to asses if this PR is going to have any further impact.

[M0stafaRady]

I'm not sure if it a standard this but I recall to notice on compiler was dealing with the `default_nettype as a global macro not just for the current file. So if we removed the line in the end of the file, the next to be compiled will be treated as if the default net type is none. So, I don't recommend it.