Remove an unnecessary end check

Author: natecook1000

Sources/_StringProcessing/Engine/MEBuiltins.swift

@@ -190,7 +190,7 @@ extension String {
     limitedBy end: String.Index,
     isScalarSemantics: Bool
   ) -> QuickResult<String.Index?> {
-    guard currentPosition < end else { return .definite(nil) }
+    assert(currentPosition < end)
     guard let (asciiValue, next, isCRLF) = _quickASCIICharacter(
       at: currentPosition, limitedBy: end
     ) else {