Remove local runner config files with --local option

[johnsudol]

Background
Currently when force removing the runner from the ui, the runner config files need to be manually removed to ensure the runner is no longer configured locally. Or the runner can be removed by using a removal token with ./config remove

Changes
Added a new option, --local, when using ./config.sh remove. This will remove the local runner files from the system without deregistering the runner from the server.

Closes #2061

Example:

 /workspaces/runner/_layout $ ./config.sh remove --local
√ Removed .credentials
√ Removed .runner