kendra was a host so she could live without a aymbiote just like sam lived without jolinar.
in the case of teal'c. junior is him immune system since if his immune system was active his body would eventually reject and attack junior. that's why teal'c can't live more than a few hours without the little bugger.
maybe the asgard can change things, noone's ever asked. the only time we've ever seen someone un-jaffa'd was jack in hathor when the sarc. fixed his newly etched pouch and healed him.
knowing what little i do about medicine, i think removing junior is beyond the sgc's abilities. it would be up to someone like the asgard or tollan to do so.
why they haven't...cause it would totally wreck the plot and his character and turn our nice strong, immune to most everything jaffa back into a regular joe and eliminate those nice things like in foothold, broca divide etc. where him immunity is a major plot point.
just like tng folk never quite humanized data.