net/netip: remove unused netIPv{4,6}len consts

net/netip: remove unused netIPv{4,6}len consts

These were added by CL 339309 but never used.

Change-Id: I40cbb5b18ac94e72bc56c15bb239677de2a202f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/521216
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>