Resolves tdf#154443 - Do not use dark shadow icons

Colibre Dark SVG icons consists of a shadow icon that draws
a dark shadow on white document canvas. Removing this solves
the issue by falling back to the Colibre standard icon (that
looks okay with dark canvas too).

Change-Id: I856a3cc89df962279cf5e1058ee8431dda0d0ea8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194960
Tested-by: Jenkins
Reviewed-by: Heiko Tietze <heiko.tietze@documentfoundation.org>

icon-themes/colibre_dark_svg/sw/res/page-shadow-mask.svg

@@ -1 +0,0 @@
-<svg height="35" viewBox="0 0 35 35" width="35" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><radialGradient id="a" cx="17.37816" cy="19.362999" gradientTransform="matrix(.85063938 -.00350568 .00364907 .88543396 2.471956 1.431619)" gradientUnits="userSpaceOnUse" r="16.5"><stop offset="0"/><stop offset="1" stop-opacity="0"/></radialGradient><path d="m0 0h35v35h-35z" fill="#fff"/><path d="m1 1v16.5 16.5h16.5 16.5v-16.5-16.5h-16.5z" fill="url(#a)"/><path d="m7 7h21v21h-21z" fill="#fafafa"/><path d="m8 8h19v19h-19z" fill="#3a3a38"/></svg>