Revert "dtrace: cache current probe in kdtrace_thread_t"

This reverts commit 22508c8b6c284ad9ad1385c703e224e1ed3fbd7c.

The t_kinst_curprobe field is no longer needed by kinst.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41031