[Sync EN] PCRE: Tidy up and clean-up some outdated things  (#4875)

Commit: `https://github.com/php/doc-en/commit/b55141fe2d4f601e32555c33fb00e1f9225c4638`
PR: `https://github.com/php/doc-en/pull/4875`

**Archivos ES a actualizar**
- `reference/pcre/book.xml`
- `reference/pcre/configure.xml`
- `reference/pcre/pattern.differences.xml`
- `reference/pcre/pattern.syntax.xml`

