Revert "net: Use fwmark reflection in PMTU discovery."

This reverts commit a3e07833.

That patch had mostly already landed upstream here:
https://android.googlesource.com/kernel/common/+/1b3c61dc1aebf5d3d6c3981ba3eedc1e66f3ecda%5E%21/



However, the documentation changes didn't make it upstream for
some reason.

I'm not sure how valuable the documentation tweak is out of tree
like this, so I wanted to propose just dropping it.

Change-Id: Ifa0904b07744c159b29880e4838bcd1245592b04
Signed-off-by: John Stultz <john.stultz@linaro.org>