[number-field] Step property should not have default value

[pekam]

Motivation

Currently, <vaadin-number-field> has step=1 by default. The default value has special functionality: it is used only for increment/decrement, but not for validation. This is quite weird and has caused some issues. See discussion in vaadin/vaadin-text-field#420 and vaadin/vaadin-text-field#421.

Actually, the native <input type="number"> validates as step would be 1, even when the property is not defined.

We can choose to either keep the current functionality (not validating by step by default), or align with the native element and do validate. Either way, having a default property value makes no sense.

Note

Part of #3102