scalar: remove stale config values

scalar: remove stale config values

These config values were added in the original Scalar contribution,
d0feac4e8c (scalar: 'register' sets recommended config and starts
maintenance, 2021-12-03), but were never fully checked for validity in
the upstream Git project. At the time, Scalar was only intended for the
contrib/ directory so did not have as rigorous of an investigation.

Each config option has its own justification for removal:

* core.preloadIndex: This value is true by default, now. Removing this
  causes some changes required to the tests that checked this config
  value. Use gui.gcwarning=false instead.

* core.fscache: This config does not exist in the core Git project, but
  is instead a config option for a Git for Windows feature.

* core.multiPackIndex: This config value is now enabled by default, so
  does not need to be called out specifically. It was originally
  included to make sure the background maintenance that created
  multi-pack-indexes would result in the expected performance
  improvements.

* credential.validate: This option is not something specific to Git but
  instead an older version of Git Credential Manager for Windows. That
  software was replaced several years ago by the cross-platform Git
  Credential Manger so this option is no longer needed to help users who
  were on that older software.

* pack.useSparse=true: This value is now Git's default as of de3a864114
  (config: set pack.useSparse=true by default, 2020-03-20) so we don't
  need it set by Scalar.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>