dtrace_test: Remove the dependency on dtraceall

FBT refuses to create probes in modules which depend on dtrace(all), but
dtrace_test is a convenient place to add functions specifically for
testing dtrace.

The dependency on dtraceall is not needed, so just remove it. In fact,
it can be useful to test SDT probe creation by loading dtrace_test with
and without dtraceall loaded.

Reviewed by: avg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46673