Remove a branch that contains redundant notebook examples

[penguian]

The notebook examples in the https://github.com/ACCESS-NRI/CMIP7-Input/tree/9-preindustrial-volcanic-forcing-for-esm16 branch have been merged into main at https://github.com/ACCESS-NRI/CMIP7-Input/tree/main/examples/esm1p6/piControl/atmosphere/volcanic so the branch should now be deleted.