Yes remove them.

I have never found a purpose for them. I do put them in a zip lock bag and lay the in the coin box just so I never loose them. It looks like some kinda ground but they touch nothing when the PF is down or up. So that's a Stern question or someone on here that can school us both.

Aren't they to minimise side to side movement of the playfield?

Yes, those are playfield shims., don't really need them.

They are there to help center / fix the play field in place when it is lowered. I remove them to install the mirrors, and then reinstall those shims over the top of the mirrors. The mounting holes are below where the mirrors end up, and if you are worried about the top of the shim scratching the mirror, don't; they land below the play field level, so impossible to see.