aegisub: do not force libiconv, use auto link flags

This fixes build after #4419.
It seems to start fine without the flags, CC maint. @AndersonTorres.
vagrant: use bin/iconv from glibc on Linux

This fixes build after #4419.
iconv: remove usage on Linux in several packages

This fixes builds after #4419. Thanks to @vbgl for the original commit;
I changed that as I'm not sure whether passing null values to buildInputs is clean.

CC maintainers: @coroa, @peti, @phreedom, @robberer, @jcumming.