Remove HookDocsGenerator legacy file

[puntope]

Changes proposed in this Pull Request:

This PR is a follow up for #2060

After adding a workflow for generating the Hook Documentation. We can remove the legacy file bin/HooksDocsGenerator.php

Detailed test instructions:

1. No references to bin/HooksDocsGenerator.php can be found in the codebase.

Additional details:

Changelog entry

Dev - Remove legacy HooksDocsGenerator.php file.

[codecov]

Codecov Report

Merging #2065 (efec923) into develop (96a5d61) will not change coverage.
Report is 5 commits behind head on develop.
The diff coverage is n/a.

❗ Current head efec923 differs from pull request most recent head da25796. Consider uploading reports for the commit da25796 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             develop   #2065   +/-   ##
=========================================
  Coverage       58.3%   58.3%           
  Complexity      4114    4114           
=========================================
  Files            454     454           
  Lines          17676   17676           
=========================================
  Hits           10297   10297           
  Misses          7379    7379           

Flag	Coverage Δ
php-unit-tests	58.3% <ø> (ø)

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[eason9487]

LGTM.

This script can be removed together:

google-listings-and-ads/package.json

Line 93 in efec923

"doc:hooks": "php bin/HooksDocsGenerator.php",