No longer accepting GSoC applications (#2743)

Update README to drop language about applying.

README.md

@@ -311,8 +311,6 @@ contribute.
     [Discord](https://discord.gg/ZjVdShJDAs).
 -   Carbon is a
     [Google Summer of Code 2023 organization](https://summerofcode.withgoogle.com/programs/2023/organizations/carbon-language).
-    [Apply to become a GSoC contributor](https://summerofcode.withgoogle.com/)
-    until April 4th.
 -   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).