We already have a LICENSE file that covers the Zig Standard Library. We
no longer need to remind everyone that the license is MIT in every single
file.
Previously this was introduced to clarify the situation for a fork of
Zig that made Zig's LICENSE file harder to find, and replaced it with
their own license that required annual payments to their company.
However that fork now appears to be dead. So there is no need to
reinforce the copyright notice in every single file.
It had the downside of running all the comptime blocks and resolving
all the usingnamespaces of each system, when just trying to discover if
the current system is a particular one.
For Darwin, where it's nice to use `std.Target.current.isDarwin()`, this
demonstrates the utility that #425 would provide.
