sgsn: Remove unused static functions from gprs_gmm.c

The following functions are not being used:
 - gsm48_tx_gmm_status_oldmsg
 - gsm48_tx_sm_status_oldmsg

This patch removes the function definitions.

Sponsored-by: On-Waves ehf
sgsn: Unassign the LLME after GMM Status without mmctx

Currently the LLME is not deleted when a GMM Status message is
received for which a mmctx cannot be found. This can fill the LLME
list with unneeded entries.

This patch adds code to unassign the LLME in that case.

Ticket: OW#1324
Sponsored-by: On-Waves ehf
gbproxy: Fixed VTY doc for delete-gbproxy-link

The command definition delete_gb_link_by_id_cmd lacks a description
for the sgsn-nsei token.

This patch adds the missing description.

Addresses:
Verifying src/gprs/osmo-gbproxy -c doc/examples/osmo-gbproxy/osmo-gbproxy.cfg, test verify_doc
Documentation error (missing docs):
<command id='delete-gbproxy-link &lt;0-65534&gt; (tlli|imsi|sgsn-nsei) IDENT'>
        <param name='IDENT' doc='(null)' />

Sponsored-by: On-Waves ehf
gbproxy/test: Extend test_gbproxy_keep_info

Add new test message sequences:
  - Normal attach (with IMSI) after detach (ok)
  - Normal attach (with local TLLI) after detach (ok)
  - Unexpected attach (with IMSI) after successful TLLI validation (fails)
  - Unexpected attach (with P-TMSI) after successful TLLI validation (fails)

Ticket: OW#1324
Sponsored-by: On-Waves ehf