Remove an excessive assert for Cairo canvas size

It is a C-style assert causing application termination when triggered.
Certain systems display pcbnew window shrinked so much, that the canvas
size is 0x0 pixels. The only problem with that is the window needs to be
resized.

Fixes: lp:1767031
* https://bugs.launchpad.net/kicad/+bug/1767031

common/gal/cairo/cairo_compositor.cpp

@@ -59,9 +59,6 @@ void CAIRO_COMPOSITOR::Resize( unsigned int aWidth, unsigned int aHeight )
 {
     clean();
 
-    assert( aWidth > 0 );
-    assert( aHeight > 0 );
-
     m_width  = aWidth;
     m_height = aHeight;