Remove unneeded checks for -1 (infinity) in SplitView

With commit 16558ac7


Layout.maximumWidth and Layout.maximumHeight now returns
Number.POSITIVE_INFINITY instead of -1.
As described in that commit, comparisons against those properties
does not need any special handling for -1 anymore.

Change-Id: I8d4bba0fa87f9c67ac30dd40a8be8cb99abba283
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>