Perf tests frequently fail saying "build not up to date"

Perf tests frequently fail saying "build not up to date"
https://bugs.webkit.org/show_bug.cgi?id=139786

Reviewed by Ryosuke Niwa.

Perf tests only build DumpRenderTree, not WebKit itself. Just clean the build
directory to get rid of stale precompiled headers, and let it rebuild
DumpRenderTree each time, that's fast.

Also, removed unused "build and perf test" code.

* BuildSlaveSupport/build.webkit.org-config/config.json:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@177529 268f45cc-cd09-0410-ab3c-d52691b4dbfc