Navigate to the game folder, most likely: C:\Program Files (x86)\steamapps\common\Total War Rome II
Copy "Rome2.exe"
Open "Launcher" folder: C:\Program Files (x86)\steamapps\common\Total War Rome II\launcher
Delete "Launcher.exe"
Paste "Rome2.exe"
Rename "Rome2.exe" to "Launcher.exe"
The Icon will automatically change. Close folder.
Return to Steam Library, select Rome II and press Play.
The game should now run without the annoying launcher.