Remove LIMIT_TX_QUEUE_SOURCE_ACCOUNT core config

[mollykarcher]

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 this config that enables/disables the 1 tx/account/ledger limit.

Why

This flag is defunct now that we're on protocol 20 (see stellar/stellar-core#3933) since it's always enabled. We aren't specifying this field anywhere anymore (https://github.com/stellar/puppet-v4/pull/3464, stellar/quickstart#485) and we've never published it externally in Horizon release notes or documentation, so doubtful any Horizon users are using it.

Known limitations

N/A

[tsachiherman]

It's good as long as the LIMIT_TX_QUEUE_SOURCE_ACCOUNT flag won't appear anywhere.
I suspect you'll need a corresponding change in the quickstart repo as well.

[mollykarcher]

I suspect you'll need a corresponding change in the quickstart repo as well.

Yeah, quickstart and puppet changes are referenced in the PR description, and are already merged