Remove runes-mode control flow block syntax validation in favor of stricter parsing

[dummdidumm]

Describe the problem

#12342 introduced validation that checks that there's no whitespace between { and #/:/@ on control flow blocks. This was done to not break backwards compatibility. We can simplify this in the next major.

Describe the proposed solution

In Svelte 6, remove the validation logic in favor of stricter parsing (so it already errors at the parser stage)

Importance

nice to have