cmd/go: don't set default GOPATH to GOROOT
cmd/go: don't set default GOPATH to GOROOT

It will just cause confusion later as the go tool will say
"warning: GOPATH set to GOROOT (%s) has no effect".
Better to just leave GOPATH unset and get that warning instead.

Change-Id: I78ff9e87fdf4bb0460f4f6d6ee76e1becaa3e7b0
Reviewed-on: https://go-review.googlesource.com/33105
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

josharian/go,Cofyc/go,polyverse-security/go,ImJasonH/go,nawawi/go,golang/go,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,CAFxX/go,linux-on-ibm-z/go,goodlang/good,vsekhar/elastic-go,polyverse-security/go,golang/go,codestation/go,goodlang/good,derekmarcotte/go,christopher-henderson/Go,suncycheng/go,CAFxX/go,golang/go,goodlang/good,Cofyc/go,ImJasonH/go,golang/go,ImJasonH/go,derekmarcotte/go,cloudflare/go,nawawi/go,akutz/go,derekmarcotte/go,vsekhar/elastic-go,derekmarcotte/go,akutz/go,suncycheng/go,linux-on-ibm-z/go,CAFxX/go,linux-on-ibm-z/go,linux-on-ibm-z/go,derekmarcotte/go,suncycheng/go,josharian/go,ImJasonH/go,josharian/go,CAFxX/go,CAFxX/go,cloudflare/go,polyverse-security/go,ImJasonH/go,codestation/go,codestation/go,codestation/go,linux-on-ibm-z/go,ImJasonH/go,ImJasonH/go,Cofyc/go,derekmarcotte/go,golang/go,goodlang/good,CAFxX/go,suncycheng/go,nawawi/go,goodlang/good,josharian/go,cloudflare/go,akutz/go,suncycheng/go,nawawi/go,ImJasonH/go,polyverse-security/go,polyverse-security/go,golang/go,codestation/go,vsekhar/elastic-go,Cofyc/go,codestation/go,akutz/go,linux-on-ibm-z/go,cloudflare/go,derekmarcotte/go,nawawi/go,nawawi/go,CAFxX/go,suncycheng/go,Cofyc/go,linux-on-ibm-z/go,akutz/go,suncycheng/go,vsekhar/elastic-go,goodlang/good,golang/go,akutz/go,josharian/go,cloudflare/go,goodlang/good,polyverse-security/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,akutz/go,codestation/go,vsekhar/elastic-go,Cofyc/go,linux-on-ibm-z/go,CAFxX/go,vsekhar/elastic-go,josharian/go,nawawi/go,goodlang/good,codestation/go,polyverse-security/go,suncycheng/go,cloudflare/go,akutz/go,derekmarcotte/go,cloudflare/go,golang/go,Cofyc/go,josharian/go,josharian/go