scalar reconfigure -a: remove stale `scalar.repo` entries

scalar reconfigure -a: remove stale `scalar.repo` entries

Every once in a while, a Git for Windows installation fails because the
attempt to reconfigure a Scalar enlistment failed because it was deleted
manually without removing the corresponding entries in the global Git
config.

In f5f0842d0b5 (scalar: let 'unregister' handle a deleted enlistment
directory gracefully, 2021-12-03), we already taught `scalar delete` to
handle the case of a manually deleted enlistment gracefully. This patch
adds the same graceful handling to `scalar reconfigure --all`.

This patch is best viewed with `--color-moved`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>