ANDROID: fix merge issue in binder_alloc_set_vma()

Upstream commit 81203ab7 ("binder: fix alloc->vma_vm_mm null-ptr
dereference") relocated the caching of alloc->vma_vm_mm to be done
during open(). This patch conflicted with local commit 0882368e
("Revert "android: binder: stop saving a pointer to the VMA"").

During the merge resolution the old assignment of alloc->vma_vm_mm
during binder_alloc_set_vma() was incorrectly left in. This shouldn't
have any implications as we check that the vma->vm_mm is the same as
current->mm. However, let's remove it for completeness.

Cc: Greg Kroah-Hartman <gregkh@google.com>
Fixes: affdbc37 ("Merge 5.15.66 into android13-5.15-lts")
Change-Id: Iea985b4f577fab71d15429fc2567b110d8174394
Signed-off-by: Carlos Llamas <cmllamas@google.com>