lxd/storage/drivers/interface: Removes "our mount" bool return value from MountVolume
lxd/storage/drivers/interface: Removes "our mount" bool return value from MountVolume

Ref counting keeps track of whether an unmount should proceed or not, so no need for indicating to caller whether they need to unmount or not.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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