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

platform/generic-arm64/rules.mk

diff --git a/platform/generic-arm64/rules.mk b/platform/generic-arm64/rules.mk
index 0d115b0..02a82ec 100644
--- a/platform/generic-arm64/rules.mk
+++ b/platform/generic-arm64/rules.mk
@@ -50,8 +50,7 @@
 MODULE_DEPS += dev/interrupt/hafnium
 MODULE_INCLUDES += \
 	external/hafnium/inc/vmapi \
-	external/hafnium/src/arch/$(HAFNIUM_ARCH)/inc \
-
+	external/hafnium/src/arch/aarch64/inc
 else
 MODULE_DEFINES += GIC_VERSION=$(GIC_VERSION)
 MODULE_DEPS += dev/interrupt/arm_gic