revert "checkout: introduce checkout.overlayMode config"

revert "checkout: introduce checkout.overlayMode config"

This reverts 1495ff7da5 ("checkout: introduce checkout.overlayMode
config", 2019-01-08) and thus removes the checkout.overlayMode config
option.

The option was originally introduced to give users the option to make
the new no-overlay behaviour the default.  However users may be using
'git checkout' in scripts, even though it is porcelain.  Users setting
the option to false may actually end up accidentally breaking scripts.

With the introduction of a new subcommand that will make the behaviour
the default, the config option will not be needed anymore anyway.
Revert the commit and remove the config option, so we don't risk
breaking scripts.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>