txnbuild: add ClearSignatures functions to Transaction and FeeBumpTransaction

[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

Add ClearSignatures functions to Transaction and FeeBumpTransaction.

Why

Transaction and FeeBumpTransaction have functions for adding signatures, but not for clearing them to get back the original transaction without signatures attached. Without these functions the only way to remove signatures is to get the envelope and fiddle with the internals of the envelope, which seems not ideal. I need these functions for some code I'm writing using the Go SDK.

Known limitations

N/A

[nikhilsaraf]

LGTM