Remove usage of deprecated thread_safe option

[alxckn]

The thread_safe option was introduced in the redis-rb's 2.2.à release. Its usage was removed in 2012 (redis/redis-rb@619fe28).

In newer versions of redis-rb (>= 5.0.0), deprecated options now result in an error raised:

Redis.new will now raise an error if provided unknown options.

This is a proposal to remove thread_safe: true in the documentation and tests as it is not used, incompatible with newer version of the redis-rb gem and in any case true is the default value.

[rosa]

Oh, totally, this must have been taken from other very old places 😅