tests: Remove plugin tests

Compiler plugins were removed in rust-lang/rust#116412, so we don't need
these tests.

As for the `plugin` field on build-targets, it appears to be
[stable-but-deprecated](https://doc.rust-lang.org/cargo/reference/cargo-targets.html#the-plugin-field),
so I left it alone

Author: epage

tests/testsuite/main.rs

@@ -132,7 +132,6 @@ mod patch;
 mod path;
 mod paths;
 mod pkgid;
-mod plugins;
 mod proc_macro;
 mod profile_config;
 mod profile_custom;