Remove verity.mk, verity_key, PRODUCT_VERITY_SIGNING_KEY

verity.mk is used to set the related variable for VB 1.0 support, but
we already removed VB 1.0. This change removes the unused code. We also
remove and block PRODUCT_VERITY_SIGNING_KEY in this change.

Bug: 241044073
Test: atest under build/make
Change-Id: Ifbcde7da27a931ef3b9d746b1c5a279d88c0ec85