Clean up build scripts

[cookpa]

@stnava @ntustison

There are some scripts in the scripts/ directory that don't seem to be used

build_itk.sh
clone_itk.sh
configure_ANTs_windows.sh
configure_ITK_External_linux.sh
configure_ITK_External_mac.sh
configure_VTK.sh
configure_VTK_External_linux.sh
configure_VTK_External_mac.sh

I was thinking to remove them but I'm not sure if they are used by other processes outside this repo. If so, perhaps we could move them to an external subdir so that it's clear what scripts are called during a normal install, which as far as I can tell are only

configure_ANTs.bat
configure_ANTs.sh
configure_ITK.bat
configure_ITK.sh

[ntustison]

FIne with me.