services/horizon: Stop generating separate Captive Core logs.

[Shaptic]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Passes along LOG_FILE_PATH="" to Stellar Core so that there is no separate log file.

Why

Closes #3438: the log file is unnecessary since Horizon already (re)logs all of Core's output as the stellar-core subservice. This also saves space in the system's /tmp directory.

Known limitations

This is not customizable.

The original issue #3438 asks for adding a parameter to customize the output log (e.g. something like --captive-core-log-path), but after the clarifications, it's unclear whether or not there's an active need in storing a separate log at all. If this is still an ask, it will be trivial to expand this PR.