Redundant toString() in string template: highlight selector only

And not the receiver. So #KT-18253 Fixed

idea/testData/inspectionsLocal/removeToStringInStringTemplate/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.RemoveToStringInStringTemplateInspection

idea/testData/inspectionsLocal/removeToStringInStringTemplate/caretInReceiver.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+
+fun foo(a: Int, b: Int) = a + b
+
+fun test(): String {
+    return "Foo: ${<caret>foo(0, 4).toString()}"
+}

idea/testData/inspectionsLocal/removeToStringInStringTemplate/caretInSelector.kt

@@ -0,0 +1,5 @@
+fun foo(a: Int, b: Int) = a + b
+
+fun test(): String {
+    return "Foo: ${foo(0, 4).<caret>toString()}"
+}

idea/testData/inspectionsLocal/removeToStringInStringTemplate/caretInSelector.kt.after

@@ -0,0 +1,5 @@
+fun foo(a: Int, b: Int) = a + b
+
+fun test(): String {
+    return "Foo: ${foo(0, 4)}"
+}