[DebugInfo] Delete unused DIVariable::getSource

Author: MaskRay

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -3230,12 +3230,6 @@ class DIObjCProperty : public DINode {
     return "";
   }
 
-  Optional<StringRef> getSource() const {
-    if (auto *F = getFile())
-      return F->getSource();
-    return None;
-  }
-
   MDString *getRawName() const { return getOperandAs<MDString>(0); }
   Metadata *getRawFile() const { return getOperand(1); }
   MDString *getRawGetterName() const { return getOperandAs<MDString>(2); }