platform: generic-arm64: Remove reference to HAFNIUM_ARCH

The Hafnium architecture on generic-arm64 is always aarch64,
so remove the reference to the HAFNIUM_ARCH so the build system
does not have to set it for us.

Bug: 122357282
Change-Id: Ida0f1476ed8708832cbc4648b7d4eee201bea93e