Revert "src/Makefile: log real/user/system time for build targets"

This broke make makeman by unconditionally adding output to all top
level targets. Lacking an obvious and trivial workaround, it makes
sense to back it out and rethink.

This reverts commit afa70490ee779646fa0e936801cb28795580bf01.

PR: 287274

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D51119