Remove triple field from triple struct in favor of str

[dtolnay]

If I have:

triple = get_host_triple(repository_ctx)

f(triple.triple)

then triple.triple does not do a good job conveying what is going on; how the two things called triple (actually 3 if you count the function named triple as well) are different from one another.

To improve this in #1289 I added a str field to the triple struct, just duplicating the contents of the triple field but with a clearer name. In contrast to triple.triple, writing f(triple.str) is clearer to me and conveys "oh yeah f takes a string argument (as opposed to a struct) and triple.str is the string representation of the triple".

This PR removes the field triple.triple in favor of triple.str since there is no longer a reason to use the former.

[UebelAndre]

Thanks for the follow-up! Looks good to me 😄