services/horizon: Remove db2/core.

[abuiles]

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

Remove db2/core.

Why

Horizon shouldn't need access to core's database.

Fix #651.

Known limitations

[2opremio]

@abuiles can you merge this into release-horizon-v1.6.0 too? (merging master into release-horizon-v1.6.0 should do)

[2opremio]

I was hoping this would remove the requirement of setting a core DB URL (and a core URL) when using captive-core, but I am still getting the following error:

Invalid config: stellar-core-db-url is blank. Please specify --stellar-core-db-url on the command line or set the STELLAR_CORE_DATABASE_URL environment variable.

[bartekn]

--stellar-core-db-url is still required in txsub to submit transactions but we don't need it when not running a webserver (ex. in reingestion context). We should allow empty values when webserver is not running.

#2779

[2opremio]

Addressed at #2780