Remove support for enketo-validate

[lindsay-stevens]

The scope would be to remove anything related to enketo-validate from pyxform. This is mainly in pyxform/validators/, but there are other references e.g. to enable the validator from xls2xform.py and in the main CLI.

Reasons:

• The pyxform_validator_update script seems to have been broken since 2020 (since this enketo-validate release).
  • The path to of xmljs.js was changed from subpath libxmljsmt to node1-libxmljsmt.
  • The updater script has no way to specify a release tag, i.e. it does not support dodging broken builds
• Enketo-validate dropped support for binary builds in 2022 in this PR as noted for release v1.15.0 in the changelog.
  • Reason: "Removed binary building as I believed it is not used and too cumbersome to fix and maintain."
  • Binary builds required so that pyxform users don't have to install node, yarn, etc
• ODK is moving from Enketo to https://github.com/getodk/web-forms
  • even if we did get it working again, it may not be relevant for very long
  • users that want to continue using enketo-validate could continue to use it separately, as they presumably are now