Since "builtin" terminfo definitions were implemented (7cbf52db1b),
the decisions made by tui.c and terminfo.c are more relevant. Exposing
that decision in the 'term' option helps with troubleshooting.
Also: remove code that allowed setting t_Co. `:set t_Co=…` has never
worked; the highlight_spec test asserting that nvim_set_option('t_Co')
_does_ work makes no sense, and should not have worked.