Cleanup Keystone and WordPressData Objective-C import

[mokagio]

In my effort to get Keystone to build for testing (Keystone scheme > Shift Cmd U) I'm running into weird build failures.

One of the things I've done to try and wrangle them is to tidy up the Objective-C imports for Keystone and WordPressData.

This PR:

• Updates all Objective-C headers to prefer forward declaration whenever possible for WordPressData, to keep the build lean
• Use modular header for WordPressData, @import WordPressData
• Remove the unused #ifdef KEYSTONE. Notice that the custom build setting had removed weeks ago via 216b4c6

[dangermattic]

	1 Warning
⚠️	View files have been modified, but no screenshot or video is included in the pull request. Consider adding some for clarity.

Generated by 🚫 Danger

[wpmobilebot]

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

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	28044
	Version	PR #24628
	Bundle ID	org.wordpress.alpha
	Commit	5ad3fbd
	Installation URL	3ojhhtvipn5co

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

[wpmobilebot]

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

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	28044
	Version	PR #24628
	Bundle ID	com.jetpack.alpha
	Commit	5ad3fbd
	Installation URL	7mq4nv2l93i1g

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

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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