[Docs] Remove UnicodeScalar from ABI/Mangling.rst (swiftlang#35785)

Follow-up to: swiftlang/swift@9e7dfad

Author: benrimmington

docs/ABI/Mangling.rst

@@ -453,7 +453,6 @@ Types
   KNOWN-TYPE-KIND ::= 'a'                    // Swift.Array
   KNOWN-TYPE-KIND ::= 'B'                    // Swift.BinaryFloatingPoint
   KNOWN-TYPE-KIND ::= 'b'                    // Swift.Bool
-  KNOWN-TYPE-KIND ::= 'c'                    // Swift.UnicodeScalar
   KNOWN-TYPE-KIND ::= 'D'                    // Swift.Dictionary
   KNOWN-TYPE-KIND ::= 'd'                    // Swift.Float64
   KNOWN-TYPE-KIND ::= 'E'                    // Swift.Encodable