Previously osmo-bts created /var/run/osmo-bts.pid which
isn't used by anything and makes it hard to run as non-root
user. Let's get rid of this pre-systemd relic.
Related: OS#4107
Change-Id: I86bcaedbc8cb1297476ad741eaa45585fea3c380
Completely drop bts_log_init(), call osmo_init_logging2() directly instead: all
callers of bts_log_init() passed NULL as category string, so all it ever did
was call osmo_init_logging(). The bts_log_info is already declared in the .h.
Here and there also define a proper talloc root context instead of using NULL.
Change-Id: Ic049f77bef74123b95350bcae182a468e0086b9c