[VSA] Remove days to completion scatterplot dropdown

Problem

• Total days to completion chart is a silly metric (#262070 (closed))
• Stage selection dropdown is not very usable: Typically users want to see data for all stages or a single stage

Solution

Updated to reflect #322126 (comment 743340733)

• After days to completion average (#262070 (closed)) is implemented...
• Remove dropdown from the chart
• The chart will be added to each stage in #346554 (closed)
• Display "Lead time" scatterplot chart in overview tab
• Display "Stage time" scatterplot chart within respective stages (e.g. Plan stage time, code stage time)