Remove seldom-used boards platforms from the primary package index

**Is your feature request related to a problem? Please describe.**

This is the flipside of arduino/arduino-ide#326.

I'm always frustrated when the Boards Manager presents me with exotic, expensive boards not many people are using. These could be added by the few users who are using them via a JSON file if needed.

As @per1234 [pointed out](https://github.com/arduino/arduino-ide/issues/326#issuecomment-821945430),

> Arduino has already made it extremely easy to install support for 3rd party boards platforms. It's no great hardship to add a URL to your preferences.

So I think only the __most popular__ boards (the ones sold by the millions) should show up there by default. Not "POC" ones very few people possess.

**Describe the solution you'd like**

Remove


because they feel like __advertising__ to me.

**Describe alternatives you've considered**

I don't know how to remove them manually.

**Additional context**

The freed-up space could be used for the most popular boards in the world, which is a separate feature request:

https://github.com/arduino/arduino-ide/issues/326

Start at the primary Boards Manager package index and inspect how the platforms shown in the issue's screenshots are represented. Confirm which entries are intended for removal, then verify that the remaining index still loads correctly and that the removed platforms can be added through a separate JSON URL.

Written by the indexing model from the issue text.