[Driver] Linux.cpp: delete unneeded D.getVFS().exists checks

Not only can this save unneeded filesystem stats, it can make `clang
--sysroot=/path/to/debian-sysroot -c a.cc` work (get `-internal-isystem
$sysroot/usr/include/x86_64-linux-gnu`) even without lib/x86_64-linux-gnu/.
This should make thakis happy.