buildperf: Avoid step usage in performance line graphs

When using steps in the line chart, you have to look long and hard
to understand if the data is the lower or upper point on the step.

Whilst not as pretty, the sloped line charts are more accurate so
switch to those as the transition points are important and the main
reason for the charts.

(From OE-Core rev: 60cc8cf421c0b04f774fd42a415f275ad457bcdf)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/build_perf/html/measurement_chart.html

@@ -82,7 +82,6 @@
       {
         name: '{{ measurement.value_type.quantity }}',
         type: 'line',
-        step: 'start',
         symbol: 'none',
         data: data
       }
@@ -137,7 +136,6 @@
       {
         name: '{{ measurement.value_type.quantity }}',
         type: 'line',
-        step: 'start',
         symbol: 'none',
         data: commitCountData
       }