SOLR-17655: Remove ExternalFileField

[epugh]

https://issues.apache.org/jira/browse/SOLR-17655

Description

ExternalFileField is deprecated, remove it.

Solution

Remove deprecated Field Type.

Tests

Existing tests

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide

[epugh]

Going to merge later this week barring any feedback!

[gerlowskija]

ExternalFileField is deprecated, remove it.

Is it? SOLR-17655 looks like it was created to put that deprecation in place, but it's not there yet afaict.

(That's not to say that we can't remove it on 'main' in anticipation of deprecating it in an eventual 9.9. I'm not saying we shouldn't remove it necessarily - just making sure we don't have wires crossed on what's deprecated at the moment.)

[epugh]

Doh, you are right: https://solr.apache.org/guide/solr/latest/indexing-guide/field-types-included-with-solr.html#deprecated-field-types doesn't actually list ExternalFileField!!!!!!!!!!!!!

If I make a seperate PR to go on branch_9x that updates the Ref Guide and adds the @Deprecated tags, does that work?