hotfix(grapher): remove missing data strategy for (stacked) discrete bar charts

[sophiamersmann]

• The transforms functions that apply the missing data strategy to DiscreteBar charts and StackedDiscreteBar charts are buggy
• Luckily, none of our DiscreteBar/StackedDiscreteBar charts in the live db currently use the missing data strategy option (maybe a sign?!)
• As a hot fix, I simply remove the data missing strategy option for these chart types
• We might add them back in the future (or maybe not, cause apparently no-one has used these options for months)
• But we might want to make Grapher a bit smarter when it comes to missing data (I'll elaborate elsewhere)