Drop the training grid's caption

Third of the explanatory lines to go, and the same case as the other two: it
told you darker meant more, which a heatmap says by being one.

The tap-to-open-that-day half went with it, but the behaviour did not — the
cells are still wired through setChartSVG. Losing the sentence costs the only
hint that they are tappable at all, which is the trade the other captions made
too.

src/index.html

@@ -142,7 +142,6 @@
         <div class="chart training-chart" id="training-chart-wrap" hidden>
           <div class="chart-title">Consistency <span data-chart-days-label>(last 7 days)</span></div>
           <svg id="chart-training" class="chart-svg" viewBox="0 0 320 60" role="img" aria-label="Training sessions per exercise per day"></svg>
-          <p class="muted-note">Darker = more sessions that day. Tap a cell to open that day.</p>
         </div>
       </section>