ANDROID: Makefile: Revert mixed-build-prefix hack

Last references were removed in http://r.android.com/3552603 and
http://r.android.com/3552602.

Since mixed-build-prefix is no longer used, we can safely remove its
definition and export.

Bug: 205893923
Test: tools/bazel build //common-modules/virtual-device:virtual_device_x86_64
Change-Id: I467d7a01648bbd079640e645e09b9f5edbdee3b3
Signed-off-by: Ulises Mendez Martinez <umendez@google.com>

Makefile

@@ -162,12 +162,7 @@ $(if $(filter-out $(words $(required_mixed_files)), \
 	$(error KBUILD_MIXED_TREE=$(KBUILD_MIXED_TREE) doesn't contain $(required_mixed_files)))
 endif
 
-mixed-build-prefix = $(if $(KBUILD_MIXED_TREE),$(KBUILD_MIXED_TREE)/)
 export KBUILD_MIXED_TREE
-# This is a hack for kleaf to set mixed-build-prefix within the execution of a make rule, e.g.
-# within __modinst_pre.
-# TODO(b/205893923): Revert this hack once it is properly handled.
-export mixed-build-prefix
 
 ifeq ("$(origin W)", "command line")
   KBUILD_EXTRA_WARN := $(W)

scripts/Makefile.modpost

@@ -38,8 +38,6 @@ __modpost:
 include $(objtree)/include/config/auto.conf
 include $(srctree)/scripts/Kbuild.include
 
-mixed-build-prefix = $(if $(KBUILD_MIXED_TREE),$(KBUILD_MIXED_TREE)/)
-
 MODPOST = $(objtree)/scripts/mod/modpost
 
 modpost-args =										\