trim.sh: Remove build-only dependencies with --depclean

Use '--with-bdeps=n' to tell Portage to ignore build-time dependencies
(e.g. an ebuild's DEPEND).  Only the runtime dependencies (an ebuild's
RDEPEND and PDEPEND) are taken into acount when selecting packages
required by @world.  That lets us keep what we need to run the target
application, but remove all the build tooling (including GCC), leaving
a functional, lightweight image.