Remove Cadence & Synopsys plugins

[harrisonliew]

ucb-bar/hammer#713 moved the (now public) Cadence & Synopsys plugins into the main Hammer repository and published package.

Also moved litex-hub channel priority lower to resolve conda package issues I was having after #1392.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[abejgonzalez]

Approved pending CI

[abejgonzalez]

Looks like some CI stalled. Restarting.

[abejgonzalez]

Looks like the setup-repo part of CI is failing/stalling. I'll just merge this in while I look at a fix.