Make Mafs line segment color offBlack64

We changed it (accidentally) to offBlack16 in
#1067. This commit fixes it. I
removed the `--mafs-line-color` custom property because it's no longer
used.

Issue: none

Test plan:

View a Mafs segment graph in the dev UI (`yarn dev`). Verify that the
segment is a darker gray than the gridlines.

packages/perseus/src/widgets/interactive-graphs/mafs-styles.css

@@ -6,8 +6,6 @@
     --mafs-bg: transparent;
     --mafs-fg: rgb(33, 36, 44); /* WB color.offBlack */
 
-    --mafs-line-color: rgba(33, 36, 44, 0.16); /* WB color.offBlack16*/
-
     --mafs-blue: #1865f2; /* WB color.blue */
     --mafs-red: #d92916; /* WB color.red */
     --mafs-green: #00a60e; /* WB color.green */
@@ -23,7 +21,7 @@
 }
 
 .MafsView .movable-segment {
-    --mafs-segment-stroke-color: var(--mafs-line-color);
+    --mafs-segment-stroke-color: rgba(33, 36, 44, 0.64);
     --mafs-segment-stroke-weight: 2px;
 }