Remove hardlink optmization in sbox

The hardlink optimization attempts to save I/O by hardlinking input
files into the sandbox instead of copying them, but it never works on
a Linux build due to nsjail always making out and the source tree into
separate bind-mounted filesystems.  If the optimization did work it
would actually cause build failures when it hardlinked the relative
bionic/libc/fs_config_generator.py symlink into the sandbox directory
without the target.  Just remove the optimization for now since it
never works.

Test: m checkbuild
Change-Id: I96e8c0c145e7c99958639594edf8a93b69ae90eb