Automatic installation of front plate mount probably depends upon the delivery center and/or the tech handling your car. As was mentioned upthread, it doesn't hurt to call the delivery center and request that they not install it. As CA is a required front plate state, you might be told that they have to install it.
If they do, it's mounted with double-sided tape so after you drive it off the lot, get some dental floss (and optionally, a heat gun / hair dryer) and go at it.
Before picking up my car last year I asked the agent (in Las Vegas) to relay my request to the Fremont, CA center to not install the mount. She told me that they have to according to CA DMV laws. When I got to the delivery center, none of the cars in the lot had them on. So YMMV.
Wouldn't be surprised if they will start to do it in CA as there might be some pressure from authorities. Literally 3/4 of the Model 3s around here and about a similar proportion of S and X don't have front plates, and police truly have better things to do than pull people over for not having front plates (for a different reason than law enforcement usually cares about).
It starts to look like Tesla owners think they're above the law for their own aesthetic whims. I say this as someone who far prefers not having a front plate, but begrudgingly does so.