emulators/{citra,yuzu}: drop runtime dependency on boost-libs

Until subpackaging arrives to bsd.port.mk and boost-libs is split into
smaller packages this shaves ~166 MiB of install size. Mainly useful
for non-qt5 versions as DEs may bring boost-libs via other routes.

(cherry picked from commit 6896626b1d2a8edb8832f5557786079338752d72)