Remove netlink code, and the PDCP_USE_NETLINK macro, as there is no
known use, it makes the code more complicated, and is simply "technical
debt".

In 4G PDCP, a subtle change has been introduced, in pdcp_module_init()
of pdcp.c. Looking at the diff of this commit, it can be seen that there
is now a new "else-if (ENB_NAS_USE_TUN_W_MBMS_BIT)" branch that is under
what was the "if (PDCP_USE_NETLINK)" branch, while the code fragment was
actually inside the "else" branch of PDCP_USE_NETLINK. This seems to be
necessary: previously, if we did not use noS1 mode and we don't use any
TUN (i.e., the eNB), the else block would set up this interface. It is
important that this interface is brought up, because if we omit it,
RFsim will not work. It seems to me that this interface will regularly
create some traffic; without it, RFsim will be blocked, and the UE
cannot connect.

In short, it seems to be important to have the MBMS interface, which
injects traffic in the RFsim side, which will unblock RFsim. The change
in pdcp_module_init() preserves the total changes of the if-else
(PDCP_USE_NETLINK).