logfields: remove unused BuildDuration and EndpointRegenerationTime constants
logfields: remove unused BuildDuration and EndpointRegenerationTime constants

As discussed in https://github.com/cilium/cilium/pull/13323, these are unused,
we don't explicitly guarantee API compatibility for them, and are not passed to
the caller across package boundaries.

The goal is to move some packages out of pkg/ to improve clarity around which
packages we provide API stability for.

Signed-off-by: Timo Beckers <98c2d5a1e48c998bd9ba9dbc53d6857beae1c9bd@isovalent.com>

cilium/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium