Remove the outdated build constraint // +build

[alexandear]

This PR removes outdated // +build comments. The "Build constraints" states:

Go versions 1.16 and earlier used a different syntax for build constraints, with a "// +build" prefix. The gofmt command will add an equivalent //go:build constraint when encountering the older syntax.

So, using only //go:build for Go 1.17+ is perfectly fine.

I used this command to remove lines starting with // +build:

find . -name '*.go' -exec sed -i '' '/^\/\/ +build/d' {} +