perf(intel): use set.difference_update in FunctionCandidate.removeCallRefs

Follow-up to the call_ref_sources list-to-set switch: collapse the
Python-level "for addr in source_addrs: discard()" loop into a single
set.difference_update() call. The semantics are identical (both no-op on
missing elements) but the work is now done in optimized C, which matters
because removeCallRefs is called from FunctionCandidateManager.updateCandidates
whenever HIGH_ACCURACY is on (the default) and conflicts are detected
during CFG recovery.

Validation:
- pytest tests/test* -> 90 passed, 43 subtests passed
- ruff check + format --check clean

Author: claude

src/smda/intel/FunctionCandidate.py

@@ -88,8 +88,7 @@ def addCallRef(self, source_addr):
         self._score = None
 
     def removeCallRefs(self, source_addrs):
-        for addr in source_addrs:
-            self.call_ref_sources.discard(addr)
+        self.call_ref_sources.difference_update(source_addrs)
         self._score = None
 
     def setIsTailcallCandidate(self, is_tailcall):