Remove RACCommand.observationInfo override

Although storing this information on the object directly is supposed to
improve performance, it seems to introduce EXC_BAD_ACCESS crashes in
some KVO methods.

Whether it’s the direct cause or just exposes some underlying issue is
unclear, but we can at least reduce the incidence by going through the
global KVO machinery.
Merge pull request #1070 from ReactiveCocoa/normal-command-kvo

Remove RACCommand.observationInfo override