apps: remove x-axis label

Remove the x-axis label.  The displayed x-axis ticks are the simplified
absolute log time for logged events and are easily correlated to log
lines.

BUG=chromium:477778
TEST=manually checked log file

Change-Id: Ib977cab0b13a1de6809445530262a1daef45ab10
diff --git a/state_graph.js b/state_graph.js
index f02dcab..0de14ad 100644
--- a/state_graph.js
+++ b/state_graph.js
@@ -156,12 +156,6 @@
         return contextY(map[d.y]);
       });
 
-  svg.append('text')
-      .attr('x', width / 2)
-      .attr('y', (focusHeight - focusMargin.bottom))
-      .style('text-anchor', 'middle')
-      .text('Elapsed Time (ms)');
-
   svg.selectAll('legend')
       .data(lineData)
       .enter()