[build_system] Don't make rm the  bin dir every time.
[build_system] Don't make rm the  bin dir every time.

mit-carbon/Graphite-Cycle-Level,fhijaz/Graphite,mit-carbon/Graphite,mit-carbon/Graphite-Cycle-Level,8l/Graphite,victorisildur/Graphite,8l/Graphite,8l/Graphite,mit-carbon/Graphite-Cycle-Level,8l/Graphite,mit-carbon/Graphite,nkawahara/Graphite,mit-carbon/Graphite,victorisildur/Graphite,mit-carbon/Graphite-Cycle-Level,victorisildur/Graphite,nkawahara/Graphite,fhijaz/Graphite,fhijaz/Graphite,fhijaz/Graphite,mit-carbon/Graphite,nkawahara/Graphite,victorisildur/Graphite,nkawahara/Graphite