Removing the final que is possible but would lead to a slight pause:

Qu’est-ce que c’est, un dîner sans vin ?

Removing que c’est from the long form will raise its formality from spoken French to standard French without substantially changing the meaning:

Qu’est-ce que c’est qu’un dîner sans vin ? (or C’est quoi, un dîner sans vin ?)

Qu’est-ce qu’un dîner sans vin ?

It is possible to go even further and write:

Qu’est un dîner sans vin ?

This is formal and literary and the meaning is also slightly stronger. Here the question is rhetorical.

Same for qu’est-ce que c’est qu’un symbole ? although qu’est un symbole ? is not very idiomatic.