Drop HOST_CROSS_ARCH special-case in source vs prebuilt mainline selection

`PRODUCT_MODULE_BUILD_FROM_SOURCE` has been added explicitly to the
products that have `HOST_CROSS_ARCH=arm64`, so we do not need this
special case here anymore.

Bug: 308188056
Test: presubmits
Change-Id: Ide3dbf9c3674683f2e0546403622cf6b8fe78739
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index cf75366..d09bcbc 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -44,12 +44,6 @@
   BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE := false
 endif
 
-# ART does not provide host side arm64 variants needed for products that
-# set HOST_CROSS_ARCH=arm64.
-ifeq (arm64,${HOST_CROSS_ARCH})
-  BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE := true
-endif
-
 ifneq (,$(MODULE_BUILD_FROM_SOURCE))
   # Keep an explicit setting.
 else ifeq (,$(filter docs sdk win_sdk sdk_addon,$(MAKECMDGOALS))$(findstring com.google.android.conscrypt,$(PRODUCT_PACKAGES))$(findstring com.google.android.go.conscrypt,$(PRODUCT_PACKAGES)))