Rollup merge of #40239 - nagisa:death-to-plugins, r=nikomatsakis

Remove ability for plugins to register a MIR pass

In recent months there have been a few different people investigating how to make a plugin that
registers a MIR-pass – one that isn’t intended to be eventually merged into rustc proper.

The interface to register MIR passes was added primarily for miri (& later was
found to make prototyping of rustc-proper MIR passes a tiny bit faster). Since miri does not use
this interface anymore it seems like a good time to remove this "feature".

For prototyping purposes a similar interface can be added by developers themselves in their custom
rustc build.

cc @nikomatsakis