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

tests/testsuite/cross_compile.rs

@@ -411,92 +411,6 @@ fn linker() {
         .run();
 }
 
-#[cargo_test(nightly, reason = "plugins are unstable")]
-fn plugin_with_extra_dylib_dep() {
-    if cross_compile::disabled() {
-        return;
-    }
-
-    let foo = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "foo"
-                version = "0.0.1"
-                authors = []
-
-                [dependencies.bar]
-                path = "../bar"
-            "#,
-        )
-        .file(
-            "src/main.rs",
-            r#"
-                #![feature(plugin)]
-                #![plugin(bar)]
-
-                fn main() {}
-            "#,
-        )
-        .build();
-    let _bar = project()
-        .at("bar")
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "bar"
-                version = "0.0.1"
-                authors = []
-
-                [lib]
-                name = "bar"
-                plugin = true
-
-                [dependencies.baz]
-                path = "../baz"
-            "#,
-        )
-        .file(
-            "src/lib.rs",
-            r#"
-                #![feature(rustc_private)]
-
-                extern crate baz;
-                extern crate rustc_driver;
-
-                use rustc_driver::plugin::Registry;
-
-                #[no_mangle]
-                pub fn __rustc_plugin_registrar(reg: &mut Registry) {
-                    println!("{}", baz::baz());
-                }
-            "#,
-        )
-        .build();
-    let _baz = project()
-        .at("baz")
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
-                name = "baz"
-                version = "0.0.1"
-                authors = []
-
-                [lib]
-                name = "baz"
-                crate_type = ["dylib"]
-            "#,
-        )
-        .file("src/lib.rs", "pub fn baz() -> i32 { 1 }")
-        .build();
-
-    let target = cross_compile::alternate();
-    foo.cargo("build --target").arg(&target).run();
-}
-
 #[cargo_test]
 fn cross_tests() {
     if !cross_compile::can_run_on_host() {

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;