vfs/idmapped-mounts: remove invalid test

vfs/idmapped-mounts: remove invalid test

The threaded mount interaction test validates that the idmapping of a
mount behaves correctly when changed while another process already has a
writable file descriptor to that mount. In newer kernels it isn't
possible to change a mount's idmapped if someone has a writable file
descriptor to that mount.

Link: e1bbcd277a53 ("fs: hold writers when changing mount's idmapping")
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>