Drop HOST_CROSS_OS special-case in source vs prebuilt mainline selection

We currently do not have many products that cross compile to
linux_bionic, and none of them use google apexes.

One exception is `mainline_sdk` product, but this product exclusively
builds using the `mainline` release config. This release config contains
the necessary RELEASE_APEX_CONTRIBUTIONS_* to provide sources during the
build, so we do not need this additional special-case here

Bug: 308188056
Test: lunch trout_arm64-next-userdebug && get_build_var
PRODUCT_PACKAGES; verified that google apexes do not appear there before
this CL, effectively making this CL a no-op for that product

Change-Id: Ibfdc742b6ae01242be22a3d3f6bb3c0cf551705f
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index a3fe2ae..ca77a52 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 linux_bionic variants needed for products that
-# set HOST_CROSS_OS=linux_bionic.
-ifeq (linux_bionic,${HOST_CROSS_OS})
-  BRANCH_DEFAULT_MODULE_BUILD_FROM_SOURCE := true
-endif
-
 # ART does not provide host side arm64 variants needed for products that
 # set HOST_CROSS_ARCH=arm64.
 ifeq (arm64,${HOST_CROSS_ARCH})