Move the clear-100x50.png image to the right directory

Also removes an unused duplicate of that file.

All the references to that file are made through
`/images/clear-100x50.png`. Since it didn't exist until now, the tests
were showing an empty expected result which confused me a bit while
debugging canvas issues.

tests/wpt/metadata/MANIFEST.json

@@ -211183,9 +211183,6 @@
    "2dcontext/building-paths/canvas_complexshapes_beziercurveto_001-ref.htm": [
     []
    ],
-   "2dcontext/clear-100x50.png": [
-    []
-   ],
    "2dcontext/compositing/2d.composite.canvas.copy.png": [
     []
    ],
@@ -211387,9 +211384,6 @@
    "2dcontext/drawing-images-to-the-canvas/drawimage_canvas_self_ref.html": [
     []
    ],
-   "2dcontext/drawing-rectangles-to-the-canvas/clear-100x50.png": [
-    []
-   ],
    "2dcontext/drawing-text-to-the-canvas/2d.text.draw.fill.basic.png": [
     []
    ],
@@ -273580,6 +273574,9 @@
    "images/canvas-line.png": [
     []
    ],
+   "images/clear-100x50.png": [
+    []
+   ],
    "images/colors.svg": [
     []
    ],
@@ -439949,10 +439946,6 @@
    "d04926ebaca2eefae7c9216d26fda5161ef6ec51",
    "reftest"
   ],
-  "2dcontext/clear-100x50.png": [
-   "eeedd0ff05889ffd4468bf19a2e8e9e0a094201c",
-   "support"
-  ],
   "2dcontext/compositing/2d.composite.canvas.copy.html": [
    "5f32421fb79c5cb96ae5e57857c47982e8502858",
    "testharness"
@@ -440937,10 +440930,6 @@
    "3e8c7d4bf236592291effa82709bb4b62bf3884e",
    "testharness"
   ],
-  "2dcontext/drawing-rectangles-to-the-canvas/clear-100x50.png": [
-   "eeedd0ff05889ffd4468bf19a2e8e9e0a094201c",
-   "support"
-  ],
   "2dcontext/drawing-text-to-the-canvas/2d.text.draw.align.center.html": [
    "85e69b06e64abbd457584ab0c0d1ed847228b39e",
    "testharness"
@@ -640977,6 +640966,10 @@
    "30f8f4540560939e1295cccb0c665b9ced908ad4",
    "support"
   ],
+  "images/clear-100x50.png": [
+   "eeedd0ff05889ffd4468bf19a2e8e9e0a094201c",
+   "support"
+  ],
   "images/colors.svg": [
    "024de84a5bbac05f0d878c994bb45d12fa41646a",
    "support"