Remove StartSpanFromContext method from ddtrace.Tracer interface to
avoid breaking users that implement this interface.

Revert tracer.StartSpanFromContext to its old behavior of inherting the
span contained in the passed context. This makes sure that users who
write tests that rely on the default NoopTracer see the same behavior as
previously.

This reverts commits 5f95c9ffa919fd1d0b660252fbec7e00de2a1b93 and
6661a9d8df77f861e1c65b9ee7c14f6f4967041e.