Kill the plugins crate and its clippy support

Sometimes clippy gets outdated by months, and its current support setup
means that each Servo component need to opt into it by depending on
the plugins crate manually, and not all components do that.

python/servo/devenv_commands.py

@@ -10,7 +10,6 @@
 from __future__ import print_function, unicode_literals
 from os import path, getcwd, listdir
 
-import subprocess
 import sys
 
 from mach.decorators import (
@@ -87,26 +86,6 @@ class MachCommands(CommandBase):
             call(["cargo", "update"] + params,
                  env=self.build_env())
 
-    @Command('clippy',
-             description='Run Clippy',
-             category='devenv')
-    @CommandArgument(
-        '--package', '-p', default=None,
-        help='Updates the selected package')
-    @CommandArgument(
-        '--json', '-j', action="store_true",
-        help='Outputs')
-    def clippy(self, package=None, json=False):
-        params = ["--features=clippy"]
-        if package:
-            params += ["-p", package]
-        if json:
-            params += ["--", "-Zunstable-options", "--error-format", "json"]
-
-        with cd(path.join(self.context.topdir, "components", "servo")):
-            return subprocess.call(["cargo", "rustc", "-v"] + params,
-                                   env=self.build_env())
-
     @Command('rustc',
              description='Run the Rust compiler',
              category='devenv')