Remove unnecessary native code

[ThexXTURBOXx]

The current native code is just boilerplate code from the initial generated plugin project skeleton.
It's time to remove it now that Flutter actively warns about using non-built-in Kotlin.

Fixes #827
Fixes #831

[adil192]

I'm all for removing boilerplate but just to make sure there's no misunderstanding, using built-in kotlin is just referring to removing the id("kotlin-android") line as outlined in https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-app-developers

[ThexXTURBOXx]

@adil192 I am aware of that - I have correctly migrated a couple of other plugins already :)
However, in this case, the native code is entirely unnecessary. Hence, it is just easier to remove it and avoid future maintenance this way as well

[jonbhanson]

@Bungeefan do you have authorization to squash and merge, or is that something that I need to do?

[Bungeefan]

@jonbhanson No, I should be able to merge them.
I just wasn't sure if I were supposed to yet (or if @ThexXTURBOXx wanted to do it himself).

[jonbhanson]

If either of you, or especially both of you, feel confident in a PR, you can feel free to merge and squash then. I actually don't feel like I'm in Flutter enough at this point to be a good judge. I'll be happy to take care of making new releases on pub.dev though. Thanks for your help!

[ThexXTURBOXx]

@Bungeefan Thank you for your reviews! I will merge my PRs after I have looked at everything myself again later.

@jonbhanson No problem at all! I/We will let you know when a new release on pub.dev would make sense.

Alas, I don't have an Apple device - hence, I cannot meaningfully review the Apple-focused PRs here :/

[ThexXTURBOXx]

@jonbhanson I forgot to add a proper changelog and update the version number in the pubspec and README. Currently, I am on my way into vacation.
After that, I will prepare all this stuff and ask you to publish a new release. Sorry about that and thank you already in advance!

[jonbhanson]

@ThexXTURBOXx no worries, I can take care of the changelog and the version number in the release that I do. I think it will actually work better that way. Just let me know when it's ready and I'll make the update and push the release.

[ThexXTURBOXx]

@jonbhanson If you prefer the workflow this way, feel free to push a new release now, actually. I had looked through my changes before my vacation and I am confident in them. Thank you very much!