Clean up relationship of metakind and node_manager (#7649)

Clean up relationship of metakind and node_manager (#7649)

Before this commit, we were including `metakind.h` twice in
`node_manager.h`, once without and once with defining
`CVC5__NODE_MANAGER_NEEDS_CONSTANT_MAP`. Additionally, `mkmetakind`
generated structs that mapped types to kinds. This commit makes all of
this obsolete by directly generating instantiations of `mkConst`, which
allows us to get rid of the double include and the `ConstantMap`.