refactor(frontends/lean): remove dead code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(builtin): remove old library, we will build a new one for Lean 0.2

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(examples/lean): remove old examples

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>