Removed asap7 hacks and updated std cell paths

[ErikFanderson]

This is an update to the pull request here. My edits:

1. Removed the methods "generate_multi_vt_gds" and "fix_icg_libs" as these methods are no longer needed with ASAP7's v28 standard cells (as you already noted).
2. Changed the tarball path to "ASAP7_PDK_CalibreDeck.tar" as this is the name of the tar file provided by ASU.
3. Updated paths to many of the std cell files from "tech-asap7-cache" to "$STDCELLS". I believe you just forgot to update all of the file paths.
4. Removed all cells from the "dont use list". This may change as I do further DRC testing.

[Waxpple]

LGTM!

Changed the tarball path to "ASAP7_PDK_CalibreDeck.tar" as this is the name of the tar file provided by ASU.

The tarball path could be different for anyone. The filename that I have originally is "calibredecks_r1p7.tar.bz2".

Removed all cells from the "dont use list". This may change as I do further DRC testing.

I only dropped the ICG cell because I am sure the original issue is solved. If you have encountered any DRC issues with any cell, you could create a new PR, please.

Thanks!