Remove support for rustc plugins

[bjorn3]

Problem

Cargo still supports building and depending on rustc plugins despite this having been removed from rustc a while ago.

Proposed Solution

Remove support for building rustc plugins.

Notes

Support was removed in rust-lang/rust#116412

[ehuss]

I think it would be good, but I think we need to be a little cautious since I believe there are people abusing (or misunderstanding) the plugin setting to force building a dylib. We may need to do a deprecation notice for a little while.

[bjorn3]

Opened #13248 for a deprecation warning.