txnbuild: remove superfluous signing key in challenge txn test

[leighmcculloch]

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 superfluous signing key in challenge transaction test that is used to sign the transaction twice when it only needs to sign the transaction once.

Why

The key is added twice which was probably the result of a copy and paste from another tests where we test the behavior of signing a transaction twice. It's unnecessary to sign it twice here and having it sign the transaction twice is noise that is irrelevant to the scenario being tested in this test.

This was reported by @overcat: lightsail-network/java-stellar-sdk#264 (comment).

Known limitations

N/A