Avoids to call checkTablesIntegrity() in checkConfig() method...

...because it throws 
```
PHP Fatal error:  Call to a member function getStorageType() on a non-object in /home/rix/bolt-comp/app/src/Bolt/Database/IntegrityChecker.php on line 618
```
in the case when you removed a previously installed extension that extends a contenttype with a custom field.

Reproducing:

* install `bolt/colourpicker`
* modify `contenttypes.yml` with adding to `pages` a colourpicker field type
* play like a child with
* remove the extension

During integrity checking Bolt wants to check db integrity but handler of the custom field will be missing.
The fix won't let run db integrity checker until improper field types has repaired in `contenttypes.yml`.