api!: remove `dc_prepare_msg` and `dc_msg_is_increation` #17357

Job	Run time
Check if node constants are up to date	4s
Lint Rust	19s
cargo deny	20s
Build C library (ubuntu-latest)	32s
Rust doc comments	19s
Build C library (macos-latest)	34s
Check provider database	5s
Build deltachat-rpc-server (ubuntu-latest)	33s
Build deltachat-rpc-server (macos-latest)	33s
Python lint	14s
Build deltachat-rpc-server (windows-latest)	19s
Rust tests (ubuntu-latest, 1.83.0)	1s
Rust tests (windows-latest, 1.83.0)	2s
Rust tests (macos-latest, 1.83.0)	21s
Rust tests (ubuntu-latest, 1.77.0)	16s
CFFI Python tests	1s
JSON-RPC Python tests	1s
	4m 34s

Provide feedback