spack flake8: remove deprecated command (#27290)

The "spack flake8" command wwas deprecated in favor
of "spack style". The deprecation wwarning is in the
0.17.X series, so removing it for v0.18.x

lib/spack/spack/cmd/flake8.py

@@ -1,25 +0,0 @@
-# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
-#
-# SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from __future__ import print_function
-
-import llnl.util.tty as tty
-
-import spack.cmd.style
-
-description = "alias for spack style (deprecated)"
-section = spack.cmd.style.section
-level = spack.cmd.style.level
-
-
-def setup_parser(subparser):
-    spack.cmd.style.setup_parser(subparser)
-
-
-def flake8(parser, args):
-    tty.warn(
-        "spack flake8 is deprecated", "please use `spack style` to run style checks"
-    )
-    return spack.cmd.style.style(parser, args)