SIL: Remove WitnessMethodInst::getSelfSubstitution()

Author: slavapestov

include/swift/SIL/SILInstruction.h

@@ -3876,12 +3876,6 @@ class WitnessMethodInst final
 
   ProtocolConformanceRef getConformance() const { return Conformance; }
 
-  /// Get a representation of the lookup type as a substitution of the
-  /// protocol's Self archetype.
-  Substitution getSelfSubstitution() const {
-    return Substitution{getLookupType(), Conformance};
-  }
-
   ArrayRef<Operand> getAllOperands() const {
     return { getTrailingObjects<Operand>(), NumOperands };
   }