Remove dead code from declarative property lookup 🤖

[lwrage]

Summary

• Remove the redundant FeatureGroup branch from declarative-model property lookup. Feature groups now reach the same inherited FeatureImpl.getPropertyValue implementation through the following Feature branch.
• Remove FeatureGroupImpl.getPropertyValueTestHelper, which has no repository caller and duplicates the inherited lookup behavior.
• Remove the empty compDecls == null block and its stale FIXME.

This is intentionally a behavior-preserving cleanup. No new regression test was added because none of the removed code could produce distinct behavior; the existing property lookup and caching tests were run before and after the change.

Fixes #3107

Validation

Focused pre-change and post-change validation:

mvn -o -T6 -s releng/osate.releng/settings.xml -Plocal \
  -pl :org.osate.aadl2,:org.osate.core.tests,:org.osate.core.feature \
  -Dtycho.localArtifacts=default \
  -Dpr.build=true -Dsign=false \
  -Dspotbugs=false -Dcodecoverage=false -Djavadoc=false \
  -Dtest=PropertyTests,PropertyCachingInstantiationTest \
  -DfailIfNoTests=false clean verify

Both runs passed 4 tests with no failures, errors, or skips.

Clean root validation:

mvn -o -T6 -s releng/osate.releng/settings.xml -Plocal \
  -Dtycho.localArtifacts=ignore \
  -Dpr.build=true -Dsign=false \
  -Dspotbugs=false -Dcodecoverage=false -Djavadoc=false \
  -DfailIfNoTests=false clean install

All 137 reactor projects succeeded. Surefire reports contained 1,429 tests with no failures, errors, or skips.

Dependencies and residual risk

There are no dependent PRs; the branch is based directly on current master at 7729ae9476.

Removing the public FeatureGroupImpl.getPropertyValueTestHelper method is binary-incompatible for an external caller of the exported implementation package. A repository-wide search found no caller, and the method never participated in property lookup, so the practical risk is limited to an unknown external consumer directly calling this unused implementation helper.