Remove --boxed and pytest-forked dependency

Fixes #468
Fixes #543

src/xdist/plugin.py

@@ -134,11 +134,6 @@ def pytest_addoption(parser):
         metavar="GLOB",
         help="add expression for ignores when rsyncing to remote tx nodes.",
     )
-    group.addoption(
-        "--boxed",
-        action="store_true",
-        help="backward compatibility alias for pytest-forked --forked",
-    )
     group.addoption(
         "--testrunuid",
         action="store",
@@ -195,14 +190,6 @@ def pytest_configure(config):
         tr = config.pluginmanager.getplugin("terminalreporter")
         if tr:
             tr.showfspath = False
-    if config.getoption("boxed"):
-        warning = DeprecationWarning(
-            "The --boxed command line argument is deprecated. "
-            "Install pytest-forked and use --forked instead. "
-            "pytest-xdist 3.0.0 will remove the --boxed argument and pytest-forked dependency."
-        )
-        config.issue_config_time_warning(warning, 2)
-        config.option.forked = True
 
     config_line = (
         "xdist_group: specify group for tests should run in same session."

src/xdist/workermanage.py

@@ -6,7 +6,6 @@ import uuid
 from pathlib import Path
 from typing import List, Union, Sequence, Optional, Any, Tuple, Set
 
-import py
 import pytest
 import execnet