Remove frustum projection mode for Camera3D

[timothyqiu]

Judging from the code, frustum projection looks the same as perspective projection + shifting the frustum, which could be made a feature of perspective projection instead of a separate mode.

https://chat.godotengine.org/channel/devel?msg=zWEfZHeOyBOrngPAt

clayjohn
I think when it was added it made sense. At that point the CameraMatrix was pretty fixed in what it could do and wasn't exposed anywhere. Now that the Projection class has been made more versatile and has been exposed it makes less sense for frustum to be its own mode.
Or even be a mode at all cause you can access the matrix directly now

The difference between frustum projection and perspective projection modes are:

1. Frustum projection shifts the frustum via frustum_offset. Perspective projection can utilize this parameter too.
2. Frustum size is specified via size, while in perspective projection it's determined by znear and fov. I think size can be exposed when using perspective projection too if needed.

Some features are also broken when using frustum projection, for example godotengine/godot#61174.

[timothyqiu]

This was proposed during the development of Godot 4.0 when compatibility breaking is still okay. It's better to keep compatibility now.