Limit options for "Delete Messages from Server" for chatmail accounts

[iequidoo]

Only the following options make sense for chatmail:

• Automatic (delete_server_after=0 i.e. the server decides when to delete messages).
• At once after Download (delete_server_after=1).

See https://github.com/deltachat/deltachat-android/pull/3147 for the reference implementation. NB: Auto-adjusting of delete_server_after isn't needed, Core >= 1.143 will do that on its own (see https://github.com/deltachat/deltachat-core-rust/pull/5840).

Probably this may be done even after switching to new Core, but better before -- at least for consistency with DC Android.

[iequidoo]

The same issue for iOS: https://github.com/deltachat/deltachat-ios/issues/2296