Remove time-unit signals

Created by: zarubaf

In order to get Verilator running we need to remove the timeunit directives. @timsaxe @gmartin102 would the below changes work for you or do you need the timeunit directives for your simulation?

One additional problem I've found is the tech-specific sram which I, for now, have removed. I would suggest we replace the entire srams with the tech_cells version we have in the repository. We do have Xilinx models automatically generated for them and for the physical implementation we can substitute those. Let me know if you think that could work, I can propose the changes here ;-)