backend: remove duplicate backend typedefs

[danbev]

This commit removes the duplicate typedefs for backend structs in ggml-backend.h. These typedefs are already defined in ggml-alloc.h which ggml-backend.h includes.

There might be a case for having the typedefs in ggml-backend.h but if this perhaps is an oversight I think it might better to remove them from ggml-backend.h to avoid confusion.