SILOptimizer: Fix a warning

Author: slavapestov

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -475,7 +475,7 @@ class ExternalFunctionDefinitionsElimination : FunctionLivenessComputation {
 
     // Check witness methods.
     for (SILWitnessTable &WT : Module->getWitnessTableList()) {
-      bool tableIsAlive = isVisibleExternally(WT.getConformance()->getProtocol());
+      isVisibleExternally(WT.getConformance()->getProtocol());
       for (const SILWitnessTable::Entry &entry : WT.getEntries()) {
         if (entry.getKind() != SILWitnessTable::Method)
           continue;