Remove the pkg/obj directory to reduce package size

GOCACHE=off no longer removes pkg/obj in go1.11beta2. Instead, follow
the go release builders and just remove the directory:

https://go.googlesource.com/build/+/779c38b4a2df669df7e9c9519a343f9ea21869a6/cmd/release/releaselet.go#56

Test: build-prebuilts.sh, check file list
Change-Id: I59164ab681a2c689f1c8a6080b6543560ad49be9