tools/logos: remove black background of U-Boot logo

When generating the Sphinx documentation with command 'make htmldocs' the
U-Boot logo is displayed with a black box as a background.

Remove the black box.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

tools/logos/u-boot_logo.svg

@@ -85,13 +85,6 @@
      inkscape:groupmode="layer"
      id="layer1"
      transform="translate(0,0)">
-    <rect
-       style="fill:#000000;fill-opacity:1;stroke:none;stroke-width:0"
-       id="rect31"
-       width="186"
-       height="186"
-       x="0"
-       y="0" />
     <circle
        style="fill:#004466;fill-opacity:1;stroke-width:0;stroke:none"
        id="path835"