Optimize imports remove reference to get extension function

#KT-3397 Fixed

idea/testData/editor/optimizeImports/multifile/ArrayAccessExpression/ArrayAccessExpression_after.kt

@@ -0,0 +1,4 @@
+import test1.MyClass
+import test1.get
+
+val s = MyClass()[1]

idea/testData/editor/optimizeImports/multifile/ArrayAccessExpression/main.kt

@@ -0,0 +1,4 @@
+import test1.MyClass
+import test1.get
+
+val s = MyClass()[1]

idea/testData/editor/optimizeImports/multifile/ArrayAccessExpression/myClass.kt

@@ -0,0 +1,8 @@
+package test1
+
+public class MyClass {
+}
+
+public fun MyClass.get(i: Int): Int {
+    return 1
+}