remove the deprecated use syntax from the language

[andrewrk]

Now that #2014 is solved, and zig fmt is auto fixing everybody's code, we can wait 1 release cycle and then delete the use keyword.

[emekoi]

commenting here because the original issue was closed. is there a specific reason that we want to discourage the use of use in code?

[hryx]

usingnamespace (néeuse) is the only language feature which gives the author the power to introduce identifiers into the current scope without a visible declaration. This has invisible side effects, crowds the symbol namespace, and generally seems counter to Zig's clarity-over-brevity virtue. I think its use warrants discouragement and the uglier name is justified.

(I still don't quite grasp why it needs to exist at all (see my unanswered question here), but the issue has been settled and this ticket is for tracking work of an already-accepted change.)

[emekoi]

my main problem with this is that is makes #1214 seem uglier, which seems to imply that it's going to be not accepted.