Removes 'PRAGMA writable_schema;'.
From sqlite changelog https://www.sqlite.org/releaselog/3_44_0.html:
> The SQLITE_DBCONFIG_DEFENSIVE setting now prevents PRAGMA writable_schema
> from being turned on.
> Previously writable_schema could be turned on, but would not actually allow
> the schema to be writable. Now it simply cannot be turned on.
Closes GH-12636.
For rationale, see #6787
Extensions migrated in part 4:
* simplexml
* skeleton
* soap
* spl
* sqlite3
* sysvmsg
* sysvsem
* tidy - also removed a check for an ancient dependency version
