[libc++][NFC] Simplify the special member functions of the node containers (#154707)

This patch does two things:
- Remove exception specifications of `= default`ed special member
functions
- `= default` special member functions

The first part is NFC because the explicit specification does exactly
the same as the implicit specification. The second is NFC because it
does exactly what the `= default`ed special member does.

GitOrigin-RevId: fd52f4d23211b0d1004f7929e5263c347b4f4399
4 files changed