simplify method of DiracDelta is deprecated

[asmeurer]

The simplify method of DiracDelta is deprecated. Use expand(diracdelta=True, wrt=x) instead.

This was done because the simplify method on other objects has a different API (no wrt flag), which was causing issues (e.g., #7716).

[mph-]

Instead of deprecating this method, I recommend having a default None value for the variable x. The expand method would be called if x is not None and self returned otherwise.

This would fix DiracDelta('t').is_constant(), and similar operations relying on the simplify method, which currently fails.

It would also make the API consistent since (3 * DiracDelta('t')).simplify() does what is expected but DiracDelta('t').simplify() fails.