[Woo] Simplify MetaData delete API

[hichamboushaba]

This PR adds 2 changes:

1. Fixes an issue with valueStrippedHtml by switching to using HtmlUtils#fastStripHtml from the utils library, the issue was discussed here.
2. Simplifies the update request by requiring only the ids of the deleted items instead of the full item, this will allow for a better usage in the app.

Testing

There is nothing really to test here, as these are just minor changes, so code review should be enough, but feel free to test the following if needed:

1. The custom fields screen in the example app (can be accessed from Woo -> Orders -> View Custom Fields)
2. This WCAndroid PR steps.

[irfano]

Tested on the WCAndroid. LGTM! 👍🏻 I'm not merging now, since you might want to make a change for my comment.