Don't suggest 'remove let' when 'it' is used multiple times

So #KT-16857 Fixed

idea/testData/intentions/replaceSingleLineLetIntention/multipleUsages.kt

@@ -0,0 +1,8 @@
+// IS_APPLICABLE: false
+// WITH_RUNTIME
+
+fun foo(s: String) {
+    if (s.substring(1).let<caret> { it.startsWith("a") || it[1].isLowerCase() }) {
+
+    }
+}