Remove removeSubrange test 5.7

Author: Azoy

test/stdlib/StringIndex.swift

@@ -1059,28 +1059,3 @@ suite.test("String.replaceSubrange index validation")
     }
   }
 }
-
-suite.test("Substring.removeSubrange entire range") {
-  guard #available(SwiftStdlib 5.7, *) else {
-    // This is a regression found in 5.7+
-    return
-  }
-
-  var a: Substring = "abcdef"
-  let aStart = a.startIndex
-  let aEnd = a.endIndex
-
-  a.removeSubrange(aStart ..< aEnd)
-
-  expectTrue(a.isEmpty)
-
-#if _runtime(_ObjC)
-  var b: Substring = ("å∫ç∂éƒ" as NSString) as Substring
-  let bStart = b.startIndex
-  let bEnd = b.endIndex
-
-  b.removeSubrange(bStart ..< bEnd)
-
-  expectTrue(b.isEmpty)
-#endif
-}