Changes

Set 'hnbap-allow-tmsi 1' by default

Due to historical reasons [0], we didn't allow UE Registration with
(P)TMSI by default, only with IMSI. The historical reasons were fixed a
long time ago, and hence why virtually every osmo-hnbgw you can find
nowadays (see doc/examples/ dir) contained an explicit "hnbap-allow-tmsi
1" to override the default behavior.

However, the fact that this was not the default behavior causes some
problems on production network which didn't have this config set
explicitly. Hence, enable it by default.

[0] Original osmo-hnbgw code in osmo-iuh.git
12181a937ff5658af49e12c57cb08ecba859e1f1 where this feature was added
explains with more detail the problem. If UE registered with TMSI, then
the HNBGW at the time didn't have information about its IMSI and hence
could not page the UE from IMSI. Since ~5.5 years ago (Sep 2019 in
osmo-sgsn.git 10b3d70fea9628eecb58f07e1bce31f63823edfb), osmo-sgsn
already sends a Common Id to the HNBGW containing the UE IMSI when it
registers, so the HNBGW has no related problem anymore.
The VTY command is left available in the event somebody wants to operate
osmo-hnbgw with a broken MSC/SGSN (like older mentioned osmo-sgsn).

We also removed tons of ue-context related logic from osmo-hnbgw in
osmo-hnbgw.git 15e552f2321979ecedb438792d507c55669e4b14, and we have
plenty of ttcn3 tests validating paging scenarios, so it doesn't seem we
have problems anymore with current code base.

Related: SYS#7430
Change-Id: I3c4c0decf34959fc2d5683b02224f27d204aa5e9