Remove BOOT_SIGNER and PRODUCT_SUPPORTS_BOOT_SIGNER

These varaibles are going to be deprecated since we removed VB 1.0
support. This change removes the related references. boot.img can be
verified by a AVB 2.0 hash descriptor now.

Bug: 241044073
Test: atest under build/make
Change-Id: I267da2d591525ffc0cabf92791cf66a36ef8ff62