Remove unused WPTableImageSource and its cache

[kean]

Removes the unused code identified in the scope of #21120.
Turned out of the additional caches I found was not even used, yay.

To test:

• Compiles

Regression Notes

1. Potential unintended areas of impact: n/a
2. What I did to test those areas of impact (or what existing automated tests I relied on): n/a
3. What automated tests I added (or what prevented me from doing so): n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr21152-a6b8389
	Version	22.8
	Bundle ID	org.wordpress.alpha
	Commit	a6b8389
	App Center Build	WPiOS - One-Offs #6441

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr21152-a6b8389
	Version	22.8
	Bundle ID	com.jetpack.alpha
	Commit	a6b8389
	App Center Build	jetpack-installable-builds #5469

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[guarani]

Thanks for removing this dead code 👍