dtrace: remove unnecessary fflush()

This call was added back in the early days of dtrace porting and
no one knows why anymore. The extra flushing causes lots of
unnecessary CPU overhead when a script produces lots of output,
as well as easily losing output because the command can't keep up.

Sponsored by: Netflix
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D34216