Remove the link to a GSoC organization (for now). (#3680)

We had some amazing GSoC participants last year, but because Carbon is
still pretty small, we ended up stetched a bit too much to be
sustainable. And this year, we're trying to have an even narrower focus
on the toolchain.

Between these aspects, we sadly don't have the bandwidth to run an
effective GSoC project this year. We think it's really important that we
can give mentees an excellent experience on the project, and don't want
to overcommit ourselves, or worse, let them down.

We're really hopeful to be back when the project is a bit larger though,
and we have good bandwidth to host folks.

README.md

@@ -355,8 +355,6 @@ contribute.
 
 -   Most of Carbon's design discussions occur on
     [Discord](https://discord.gg/ZjVdShJDAs).
--   Carbon is a
-    [Google Summer of Code 2023 organization](https://summerofcode.withgoogle.com/programs/2023/organizations/carbon-language).
 -   To watch for major release announcements, subscribe to our
     [Carbon release post on GitHub](https://github.com/carbon-language/carbon-lang/discussions/1020)
     and [star carbon-lang](https://github.com/carbon-language/carbon-lang).