cmd/go/internal/get: remove unused tag selection code
cmd/go/internal/get: remove unused tag selection code

selectTag has been hard coded to only understand the tag `go1` since
CL 6112060 which landed in 2012. The commit message asserted;

  Right now (before go1.0.1) there is only one possible tag,
  "go1", and I'd like to keep it that way.

Remove goTag and the unused matching code in selectTag.

Change-Id: I85f7c10f95704e22f8e8681266afd72bbcbe8fbd
Reviewed-on: https://go-review.googlesource.com/38112
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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