Revert "[cxx-interop] Add conversions between std::u16string and Swift.String"

Author: egorzhdan

stdlib/public/Cxx/std/String.swift

@@ -10,8 +10,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// MARK: Initializing C++ string from a Swift String
-
 extension std.string {
   public init(_ string: String) {
     self.init()
@@ -21,31 +19,12 @@ extension std.string {
   }
 }
 
-extension std.u16string {
-  public init(_ string: String) {
-    self.init()
-    for char in string.utf16 {
-      self.push_back(char)
-    }
-  }
-}
-
-// MARK: Initializing C++ string from a Swift String literal
-
 extension std.string: ExpressibleByStringLiteral {
   public init(stringLiteral value: String) {
     self.init(value)
   }
 }
 
-extension std.u16string: ExpressibleByStringLiteral {
-  public init(stringLiteral value: String) {
-    self.init(value)
-  }
-}
-
-// MARK: Initializing Swift String from a C++ string
-
 extension String {
   public init(cxxString: std.string) {
     let buffer = UnsafeBufferPointer<CChar>(
@@ -56,12 +35,4 @@ extension String {
     }
     withExtendedLifetime(cxxString) {}
   }
-
-  public init(cxxU16String: std.u16string) {
-    let buffer = UnsafeBufferPointer<UInt16>(
-      start: cxxU16String.__dataUnsafe(),
-      count: cxxU16String.size())
-    self = String(decoding: buffer, as: UTF16.self)
-    withExtendedLifetime(cxxU16String) {}
-  }
 }

test/Interop/Cxx/stdlib/overlay/std-string-overlay.swift

@@ -37,36 +37,6 @@ StdStringOverlayTestSuite.test("std::string <=> Swift.String") {
   expectEqual(swift6, "xyz\0abc")
 }
 
-StdStringOverlayTestSuite.test("std::u16string <=> Swift.String") {
-  let cxx1 = std.u16string()
-  let swift1 = String(cxxU16String: cxx1)
-  expectEqual(swift1, "")
-
-  let cxx2 = std.u16string("something123")
-  expectEqual(cxx2.size(), 12)
-  let swift2 = String(cxxU16String: cxx2)
-  expectEqual(swift2, "something123")
-
-  let cxx3: std.u16string = "literal"
-  expectEqual(cxx3.size(), 7)
-
-  let cxx4: std.u16string = "тест"
-  expectEqual(cxx4.size(), 4)
-  let swift4 = String(cxxU16String: cxx4)
-  expectEqual(swift4, "тест")
-
-  // Emojis are represented by more than one CWideChar.
-  let cxx5: std.u16string = "emoji_🤖"
-  expectEqual(cxx5.size(), 8)
-  let swift5 = String(cxxU16String: cxx5)
-  expectEqual(swift5, "emoji_🤖")
-
-  let cxx6 = std.u16string("xyz\0abc")
-  expectEqual(cxx6.size(), 7)
-  let swift6 = String(cxxU16String: cxx6)
-  expectEqual(swift6, "xyz\0abc")
-}
-
 extension std.string.const_iterator: UnsafeCxxInputIterator {
   // This func should not be required.
   public static func ==(lhs: std.string.const_iterator,