[stdlib] Remove @_specialize attributes obsoleted by explicit type checks

Author: lorentey

stdlib/public/core/StringBridge.swift

@@ -791,8 +791,6 @@ extension StringProtocol {
     return self.utf16.index(self.utf16.startIndex, offsetBy: offset)
   }
 
-  @_specialize(where Self == String)
-  @_specialize(where Self == Substring)
   public // SPI(Foundation)
   func _toUTF16Offsets(_ indices: Range<Index>) -> Range<Int> {
     if Self.self == String.self {
@@ -808,8 +806,6 @@ extension StringProtocol {
     return Range(uncheckedBounds: (lower: startOffset, upper: endOffset))
   }
 
-  @_specialize(where Self == String)
-  @_specialize(where Self == Substring)
   public // SPI(Foundation)
   func _toUTF16Indices(_ range: Range<Int>) -> Range<Index> {
     if Self.self == String.self {