Remove never intentionally used String.hashCode extension.

compiler/testData/codegen/boxWithStdlib/boxingOptimization/kt5493.kt

@@ -3,6 +3,6 @@ fun box() : String {
         return "OK"
     }
     finally {
-        null?.hashCode()
+        null?.toString()
     }
 }

libraries/stdlib/src/kotlin/text/StringsJVM.kt

@@ -46,12 +46,6 @@ public fun String.equals(anotherString: String, ignoreCase: Boolean = false): Bo
     else
         (this as java.lang.String).equalsIgnoreCase(anotherString)
 
-/**
- * Returns the hash code of this string.
- */
-public fun String.hashCode(): Int = (this as java.lang.String).hashCode()
-
-
 /**
  * Returns a copy of this string with all occurrences of [oldChar] replaced with [newChar].
  */