Support: remove extension that is unused

Clean up the unused extension for `String` as work towards removing the
UTF16 string extensions.

Author: compnerd

Sources/SwiftWin32/Support/String+UIExtensions.swift

@@ -1,14 +1,6 @@
 // Copyright © 2019 Saleem Abdulrasool <compnerd@compnerd.org>
 // SPDX-License-Identifier: BSD-3-Clause
 
-extension String {
-  internal init(from utf16: [UInt16]) {
-    self = utf16.withUnsafeBufferPointer {
-      String(decodingCString: $0.baseAddress!, as: UTF16.self)
-    }
-  }
-}
-
 extension String {
   public var wide: [UInt16] {
     return Array<UInt16>(from: self)