Revert "net: add sysctl accept_ra_min_rtr_lft"

This reverts commit 354a9677 which is
commit 1671bcfd upstream.

It breaks the Android ABI, and is already merged in the non-LTS branch
in an abi-safe way.

Bug: 161946584
Change-Id: Ie689c535c1f7c1c4a1e6af6805f400da625104c7
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>