build: moving deadcode

[andresvia]

Purpose

There is small piece of dead code in build.go as a consequence building on go < 1.5 panics, instead of log.Fatalf with friendly message.

Testing

eval $(gimme 1.4)
go get ./...
go run build.go

[calmh]

I'm was under the impression Syncthing didn't build at all on anything less than 1.5 anymore, making this a moot point - and the vendor check should be removed really, as a consequence.

[andresvia]

Indeed, removing the go vendor availability check as this is already covered by the min version check.

[calmh]

@st-review merge it

build: Remove obsolete check for vendoring support

Skip-check: authors

[st-review]

👌 Merged as 566c348. Thanks, @andresvia!