dashboard: fix update script It needs to remove perf results rather than build results.
dashboard: fix update script
It needs to remove perf results rather than build results.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/164000043

golang/build,proppy/build,evandbrown/build,evandbrown/build,golang/build,proppy/build,golang/build,proppy/build,proppy/build,golang/build,evandbrown/build