Remove deprecated android-extensions plugin.

[madsager]

It has been deprecated for a long time and is scheduled for removal in 1.8.

See https://android-developers.googleblog.com/2022/02/discontinuing-kotlin-synthetics-for-views.html

[madsager]

Getting started on this work to make sure that we have enough time to clean it up properly. My guess would be that this might break things downstream (IntelliJ/Android Studio). I'll check what happens in an intellij community build using this next week. If you have ideas on what else could break, do let me know so I can get started on fixes. :)

[udalov]

I've reported an issue here for visibility: https://youtrack.jetbrains.com/issue/KT-54013/Remove-deprecated-Kotlin-Android-Extensions-compiler-plugin