[!!!][TASK] Reduce size of reference index

[rr-it]

This excludes the luxletter link, log and queue tables from the reference index.
Thereby the size of the reference index is massively reduced.

For this patch to become effective it is necessary to manually run:

• SQL:

  DELETE FROM `sys_refindex`
  WHERE `tablename` IN (
    'tx_luxletter_domain_model_link',
    'tx_luxletter_domain_model_log',
    'tx_luxletter_domain_model_queue'
  );

• vendor/bin/typo3 referenceindex:update

This fixes #130

[einpraegsam]

Thx for digging into it. 2 things just come into my mind:

1. I would like to not use final for class. That is fine for project development but not for a community extension that could be extended from anyone
2. I would also like to use the same code style as it is already implemented in LUX - see https://github.com/in2code-de/lux/blob/develop/Classes/TCA/PreventReferenceIndex.php
   Because both extensions are similar created I would like to have a similar codebase

[rr-it]

Refactored to use LUX code style

[einpraegsam]

Thx for your work