.flake8 in azure-cli repo is not honored

[jiasli]

.flake8 in azure-cli repo is not honored.

azdev is using its own flake8 config file:

azure-cli-dev-tools/azdev/operations/style.py

Lines 184 to 185 in d3c663e

	cli_result = run(cli_paths, 'cli.flake8', 'modules')
	ext_result = run(ext_paths, 'ext.flake8', 'extensions')

We need to disable one of them and centralize to one file.

This causes the CI failure in Azure/azure-cli#13844 even after E402 is added to .flake8:

/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:13:1: E402 module level import not at top of file
/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:14:1: E402 module level import not at top of file
/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:16:1: E402 module level import not at top of file
/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:17:1: E402 module level import not at top of file
/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:18:1: E402 module level import not at top of file
/home/vsts/work/1/s/src/azure-cli/azure/cli/__main__.py:19:1: E402 module level import not at top of file
6     E402 module level import not at top of file