Replace make ext target and its documentation

With the ext/ subdirectories being an integral part
of the repo, as they were added with git subtree,
there is no need to init/deinit/clean etc. When
this repo is checked out, the test and build dependencies
are also checked out.

make ext reads the file ext/dependencies.lisp-expr and
performs a git subtree pull for all packages in ext to
update them to the specified treeish (use 'master' to
update to the latest HEAD)