If you have R2015a or later, and if your values are all numeric and the same size, then
to_delete_mask = ismembertol(cell2mat(YourCell), [-99.9, -99.9], 'ByRows', true);
YourCell(to_delete_mask,:) = [];
If you are using an earlier release you will need to test abs() of the difference to -99.9 against a small tolerance.
It may appear that you should be able to just compare your values to -99.9 but you cannot do so because of floating point precision roundoff; what looks like exactly -99.9 is not exactly that because it is not possible to exactly represent the fraction 9/10 in finite binary floating point.
