chore: clean up PR template (#13735)

time to remove the Svelte 5 note

Author: dummdidumm

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,3 @@
-## Svelte 5 rewrite
-
-Please note that [the Svelte codebase is currently being rewritten for Svelte 5](https://svelte.dev/blog/runes). Changes should target Svelte 5, which lives on the default branch (`main`).
-
-If your PR concerns Svelte 4 (including updates to [svelte.dev.docs](https://svelte.dev/docs)), please ensure the base branch is `svelte-4` and not `main`.
-
 ### Before submitting the PR, please make sure you do the following
 
 - [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs