linkbench: Remove do_both mode
linkbench: Remove do_both mode

We don't use this, it's not clear it's even meaningful, and it
complicates the code.

aclements/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6