message: remove the judgy comment regarding spin bit
  Summary:
  as title
  (Note: this ignores all push blocking failures!)
diff: @@ -691,9 +691,8 @@ struct ShortHeaderInvariant {
  struct ShortHeader {
  public:
  + // There is also a spin bit which is 0x20 that we don't currently implement.
  static constexpr uint8_t kFixedBitMask = 0x40;
  - // There is also a spin bit which is 0x20, but as a decent implementation of
  - // course we don't implement that.
  static constexpr uint8_t kReservedBitsMask = 0x18;
  static constexpr uint8_t kKeyPhaseMask = 0x04;
  static constexpr uint8_t kPacketNumLenMask = 0x03;
  