Remove mostly-useless proc provider probes.

For some reason the proc UMA zone's ctor, dtor and init functions are
instrumented, but these functions are always available through FBT.
Moreover, the probes are not part of the original Solaris proc
provider, aren't documented, have no uses (e.g., in dwatch(8)) and
have no clear use to begin with. Therefore, remove them.

Reviewed by: rpaulo
Differential Revision: https://reviews.freebsd.org/D2169