Remove Quat set methods in favour of constructors

[madmiraal]

godot/core/variant/variant_call.cpp

Lines 1129 to 1131 in d39f638

	// FIXME: Quat is atomic, this should be done via construcror
	//ADDFUNC1(QUAT, NIL, Quat, set_euler, VECTOR3, "euler", varray());
	//ADDFUNC2(QUAT, NIL, Quat, set_axis_angle, VECTOR3, "axis", FLOAT, "angle", varray());

A Quat should be immutable. It should not have set_* methods. When required, a new Quat should be created using the equivalent constructor.

This PR removes the Quat set_* methods, and, where used, replaces them with the constructor equivalents. Note: This PR does not make Quat immutable. It's components are still public and they can still be (and are) changed within the engine.

For reference, deprecating them was originally suggested by @aaronfranke here as part of #16863.

[vnen]

Please don't change GDNative before #44989 is merged, otherwise conflicts will haunt me. I'm removing those methods there anyway.

[akien-mga]

Please don't change GDNative before #44989 is merged, otherwise conflicts will haunt me. I'm removing those methods there anyway.

That PR is now merged.

[akien-mga]

Looks good to me (pending rebase for GDNative).

[madmiraal]

Rebased.

[akien-mga]

Thanks!