Don't debug SystemId's entity field twice

[SkiFire13]

Objective

• SystemId's Debug implementation includes its entity field twice.
• This was likely an oversight in Allow Commands to register systems #11019, since before that PR the second field was the PhantomData one.

Solution

• Only include it once

Alternatively, this could be changed to match the struct representation of SystemId, thus instructing the formatter to print a named struct and including the PhantomData field.