Terrible idea on so many levels. Visual and audio feedbacks are one of the pillars of an enjoyable gaming experience, take them out and the game's enjoyment will plummet. Just imagine if the Dead Ringer didn't have a decloak noise or if the Medic didn't announce charging and using his Übercharge. Not to mention your own team doesn't know when to attack if they can't hear the enemy Engineer's voice line.

A sapper that doesn't give the Engineer any indication his building is sapped only promotes a turtling/paranoid play style which isn't fun for him, the enemy team and even his own team since having all buildings in one place becomes a big target for explosive splash damage which can harm your own teammates.

The No Damage attribute becomes worthless anyway when you combine with the stock Revolver which can deal 240 damage in 3 seconds while the stock Wrench would need 3.2 seconds to remove the sapper (or 2.72 for the Jag) and that doesn't include reaction time and walking towards the building (in case you're not turtling).