"veyron-browser": Remove new_security_model flag
"veyron-browser": Remove new_security_model flag

Also see: https://veyron-review.googlesource.com/#/c/6244/

Change-Id: Idc0f22098a602fd90f6b68f9bb93aea73b753c8f

vanadium/browser,vanadium/browser,vanadium/browser,vanadium/browser