Clear template cache when plugin deactivated

[jakejackson1]

Description

When ever the plugin is deactivated the PDF template cache transients will be deleted.

Resolves #1507

Testing instructions

1. Deactivate Gravity PDF
2. Install custom template via FTP
3. Reactivate Gravity PDF
4. Open PDF Template Manager and locate template installed in step 2

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 76.96%. Comparing base (3df0d31) to head (1f9d7f0).

Files	Patch %	Lines
src/Controller/Controller_Activation.php	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1512 +/- ## =============================================== - Coverage 76.97% 76.96% -0.02% =============================================== Files 244 244 Lines 12740 12742 +2 Branches 370 370 =============================================== Hits 9807 9807 - Misses 2925 2927 +2 Partials 8 8 ```

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