ANDROID: Drop mixed-build-prefix from Makefile.modinst.

This partially reverts the conflict resolution done by commit
51786e2d ("Merge 61401a87 ("Merge
tag 'kbuild-v6.6' of git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild"

)
into android-mainline").

Now that KBUILD_MIXED_TREE is identical to objtree, there is no point
of prepending mixed-build-prefix here.

Bug: 402231876
Change-Id: Iac8b1fd4fc6a55d71f97b5ae71afb52feceb168d
Signed-off-by: HONG Yifan <elsk@google.com>