ANDROID: Drop second argument from depmod.sh.

This partially reverts commit ee1b16e8
("ANDROID: Kbuild: Add support for KBUILD_MIXED_TREE").

Now that with commit 214c0eea
("kbuild: add $(objtree)/ prefix to some in-kernel build artifacts"),
System.map is found in $objtree already, so there is no need to set
mixed-build-prefix any more.

Bug: 402231876
Change-Id: Ic48bd2f48ea41dd086e72a9071921ef2a116b8a4
Signed-off-by: HONG Yifan <elsk@google.com>