detect: remove lock from global keyword logic

detect: remove lock from global keyword logic

The global keyword registration and per thread init handling used
the lock from the DetectEngineMasterCtx. This lead to a dead lock
situation at multi-tenancy tenant reloads.

The lock was unnecessary however, as the only time the registration
list is updated is at engine initialization. At that time Suricata
is still running in a single thread. After this, the data structure
doesn't change anymore.

Bug #2516.