docs: remove stale missing regex heading

The audited regex entries are capability records rather than a list of
missing features, so keep them in the main regular-expression section.

Generated with Codex (https://openai.com/codex/)

Co-Authored-By: Codex <noreply@openai.com>

Author: fglock

docs/reference/feature-matrix.md

@@ -385,8 +385,6 @@ my @copy = @{$z};         # ERROR
 - ✅  **Preprocessor**: `\Q`, `\L`, `\U`, `\l`, `\u`, `\E` are preprocessed in regex.
 - ✅  **Overloading**: `qr` overloading is implemented. See also [overload pragma](#pragmas).
 
-### Missing Regular Expression Features
-
 - ✅  **Dynamically-scoped regex variables**: Provisional captures, `$^R`, `$^N`, match positions, and callback locals follow matcher paths and unwind on backtracking.
 - ✅  **Recursive and Dynamic Patterns**: `(?R)`, `(?0)`, and runtime `(??{ code })` execute through Joni. Dynamic expressions may return strings or `qr//` values, nested alternatives participate in outer backtracking without changing outer grouping or capture numbering, and callback and pure-pattern recursion have engine-owned depth ceilings.
 - ✅  **Backtracking Control Verbs**: `(*ACCEPT)`, `(*FAIL)`/`(*F)`, `(*PRUNE)`, `(*SKIP)`, `(*THEN)`, and `(*COMMIT)` execute through Joni with matcher-owned cut boundaries. Atomic groups `(?>...)` are supported.