Configure cmake-language-server and coc-cmake

Utilises the following Python package:

```console
$ pip3 install --user cmake-language-server
```

after/ftplugin/cmake.vim

@@ -1,6 +1,3 @@
-" Add omnifunc completion plugin.
-packadd vim-cmake-completion
-
 " Enable syntax folding, see .vim/after/syntax/cmake.vim
 setlocal foldmethod=syntax
 " Set comment string