box: drop space_id for FK referring to same space

It is inconvenient to create self-referencing FK constraints, as the
space ID will only be generated during space creation. This is
especially useful for SQL, since the format for the space is created
at compile time, and the space ID is only obtained at run time.

Closes #7200

@TarantoolBot document
Title: Describe foreign keys referring to the same space
Since: 2.11
Root document: https://www.tarantool.io/en/doc/latest/book/box/data_model/#foreign-keys

It is possible to create a foreign key that refers to the same space (a
child space equals to the parent space).
To do that, omit `space` in the `foreign_key` parameter, or set it to
the id or to the name of the current space.