GPU Python: Use string literals in shader.from_builtin and shader.code_from_builtin.

Also, the gpu.shader.builtin submodule becomes obsolete, so it has been removed.

source/blender/python/gpu/gpu_py_shader.h

@@ -37,6 +37,5 @@ typedef struct BPyGPUShader {
 
 PyObject *BPyGPUShader_CreatePyObject(struct GPUShader *shader, bool is_builtin);
 PyObject *BPyInit_gpu_shader(void);
-PyObject *BPyInit_gpu_shader_builtin(void);
 
 #endif /* __GPU_PY_SHADER_H__ */