net-p2p/*ktorrent: tidy up dependencies

• Boost is a build-dep (wasn't listed explicitly for ktorrent, because it was dragged in implicitly by other KDE frameworks),
• ECM is a build-dep (either listed as runtime-dep, or missing and implicit from being a runtime-dep for something else).

While here, remove comment about keeping things in sync which
is no longer relevant since ktorrent and libktorrent are part
of the KDE release service.