Agent build system clean task: remove only agent build products, not the entire buildout/support-binaries
Agent build system clean task: remove only agent build products, not the entire buildout/support-binaries

Removing the entire buildout/support-binaries directory removes too many things
and breaks the Homebrew native packaging tests. The Homebrew native packaging tests
run passenger-install-apache2-module and passenger-install-nginx-module, each of which
call 'rake common:clean'. Prior to this commit, that Rake task removes
buildout/support-binaries/nginx-* (the Passenger Standalone Nginx engine) too,
which breaks the Homebrew Passenger Standalone tests.

phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger,phusion/passenger