With the introduction of X2AP into develop, the UEs now have to regularly
send measurement reports.

In the logs of the eNB, we see:

[OSA]   Mismatch found in integrity for algorithm 2,
        got e0.a0.c2.66, expecting a5.9c.cb.57
[PDCP]   [OSA][RB 1] eNB failed to validate MAC-I of incoming PDU

This is a bug in the PDCP layer that uses wrong parameters to compute the
integrity.

This commit fixes this bug.

The function pdcp_is_rx_seq_number_valid was removed. Its processing
has been directly integrated into the function pdcp_data_ind.

The function pdcp_mark_current_pdu_as_received is not called anymore.
Its processing was not used later on, so as of today, not calling it does
not introduce any functional change.

The function pdcp_validate_security takes now as parameters both
SN and HFN. Same for the function pdcp_get_next_count_rx.

Useless constants PDCP_SN_5BIT, PDCP_SN_7BIT and PDCP_SN_12BIT have been
removed.

The compilation option ENA...