PyAPI: remove GPUVertFormat() creation from a list

We already have `attr_add` method,
best not have two ways to do the same thing.

See: D3830

source/blender/python/gpu/gpu_py_vertex_format.h

@@ -37,7 +37,5 @@ typedef struct BPyGPUVertFormat {
 } BPyGPUVertFormat;
 
 PyObject *BPyGPUVertFormat_CreatePyObject(struct GPUVertFormat *fmt);
-bool bpygpu_vertformat_from_PyList(
-        const PyListObject *list, const char *error_prefix, GPUVertFormat *r_fmt);
 
 #endif /* __GPU_PY_VERTEX_FORMAT_H__ */