Remove _collapse_degenerate_points_and_bounds() call in pp_rules

**DO NOT MERGE**

This PR removes the logic the in the new `_convert_time_coords()` that collapses any degenerate dimensions. It may be the case that this logic is redundant as the collapsing/reshaping is done by the field collation code. This would be good as this code uses `np.allclose` because of floating point issues with netcdf datetimes, which I would like to avoid.

Note it may be the case that the way in which the t1, t2 and lbft arrays are combined may mean that non-degenerate metadata vectors produce coordinates with degenerate values, in which case this code may be required/desirable.

I suggest we get the end to end pipeline working and then investigate using this PR.