Remove discriminant symbols.

Given that bootstrapping and running the testsuite works without
exporting discriminant values as global constants, I conclude that
they're unused and can be removed.
auto merge of #8546 : jld/rust/discrim-symbol-rm, r=pcwalton

Given that bootstrapping and running the testsuite works without
exporting discriminant values as global constants, I conclude that
they're unused and can be removed.