Remove incorrect stimuli modality mapping

Stimuli are modality-agnostic per BEP044 specification and should
not be mapped to a specific modality. Remove the stim modality
mapping to fix schema validation errors.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: neuromechanist

src/schema/rules/modalities.yaml

@@ -34,6 +34,3 @@ nirs:
 mrs:
   datatypes:
     - mrs
-stim:
-  datatypes:
-    - stimuli