make : remove make in favor of CMake (ggml-org#15449)

This commit removes the content from the Makefile and updates the
current deprecation message to information that `make` has been
replaced by CMake instead.

The message when `make` is invoked will now be the following:
```console
$ make
Makefile:6: *** Build system changed:
 The Makefile build has been replaced by CMake.

 For build instructions see:
 https://github.com/ggml-org/llama.cpp/blob/master/docs/build.md

.  Stop.
```

The motivation for this is that many, if not all targets fail to build
now, after changes to the system, and `make` has also been deprected for
some time now.

Author: danbev