Change Signature: Use correct index in getOriginalParameter()

#KT-18642 Fixed
 #KT-18606 Fixed

idea/testData/quickfix/changeSignature/removeParameterFromExtension.kt

@@ -0,0 +1,4 @@
+// "Remove parameter 'a'" "true"
+
+fun String.foo(<caret>a:Int = 0, b: Int) {
+}

idea/testData/quickfix/changeSignature/removeParameterFromExtension.kt.after

@@ -0,0 +1,4 @@
+// "Remove parameter 'a'" "true"
+
+fun String.foo(b: Int) {
+}