COMP: Fix compiler warnings with ITK_USE_SYSTEM_VXL

Commit 41793b40 attempted to solve an unused variable warning by forking
VXL. The warning was still triggered when ITK_USE_SYSTEM_VXL was enabled.

This reverts the fork of VXL and solves the warning when using system VXL
too.

Change-Id: I61db91d3269818a4819541dc5ec6a25d273f3dd2
Merge topic 'VNL-Diag-Warning'

0cfabd0 COMP: Fix compiler warnings with ITK_USE_SYSTEM_VXL