Refactor: kill BLS12-378 and BW6-756 curves

[yelhousni]

Description

This PR kills BLS12-378 and BW6-756 curves support. These are GT-strong 2-chains, i.e. no need to check if GT elements are of order r. This is useful for something like SNARKPack+composition, or anything that uses GT ops (e.g. SSLE). This was part of an external collaboration which never came into being.

Type of change

• This change requires a documentation update

How has this been tested?

N/A

How has this been benchmarked?

N/A

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules