Remove EngineFeature#SCAN_NEEDS_SIGNATURE

### Description

#15914 introduced some enhancements which have integrated what `EngineFeature#SCAN_NEEDS_SIGNATURE` was designed for more deeply - essentially making it mostly redundant.

It was marked as deprecated as the removal seemed like to raise some of its own questions.

### Motivation

Reduce complexity :)

Start by locating EngineFeature#SCAN_NEEDS_SIGNATURE and reviewing the enhancements from #15914 that made it redundant. Trace its references and related tests, then remove the deprecated feature and update the affected references so the project still builds and tests pass.

Written by the indexing model from the issue text.