box: drop 'execute' field from uninitialized box

Prior to this patch, it was possible to call box.execute() before box
was initialized, i.e. before calling box.cfg(). This, however, caused
box.cfg() to be called automatically, which could be problematic as some
parameters could not be changed after box.cfg() was called. After this
patch, box.execute() will only be available when the box has been
initialized.

Closes #4726

@TarantoolBot document
Title: box.execute() now available only after initialization of box

Previously, it was possible to call box.execute() before the box was
configured, in which case the box was configured automatically, which
could lead to problems with box parameters. Now box.execute() can only
be called after the box has been properly configured.

It is also forbidden to set language to SQL in a console with an
unconfigured box.