fix: Remove legacy code from Django pre 1.4

[fsbraun]

This PR is required for Django 5

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.18% :tada:

Comparison is base (10de493) 76.18% compared to head (c3d01fd) 76.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #651 +/- ## ========================================== + Coverage 76.18% 76.36% +0.18% ========================================== Files 20 20 Lines 932 918 -14 Branches 124 123 -1 ========================================== - Hits 710 701 -9 + Misses 196 191 -5 Partials 26 26 ``` | [Files Changed](https://app.codecov.io/gh/django-cms/djangocms-text-ckeditor/pull/651?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms) | Coverage Δ | | |---|---|---| | [djangocms\_text\_ckeditor/utils.py](https://app.codecov.io/gh/django-cms/djangocms-text-ckeditor/pull/651?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZGphbmdvY21zX3RleHRfY2tlZGl0b3IvdXRpbHMucHk=) | `94.11% <ø> (+4.21%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.