Revert vgo support

Reverts #562 where we added support for vgo.

I'd tested vgo's behavior on a simple synthetic versioned repository
before merging #562, but apparently it wasn't enough because I'm unable
to correctly add stripe-go with vgo even with the addition of a `go.mod`
and corresponding tag.

Here's the resolution error when trying to build a test project:

```
$ vgo build
vgo: resolving import "github.com/stripe/stripe-go/v30"
vgo: finding github.com/stripe/stripe-go/v30 (latest)
vgo: adding github.com/stripe/stripe-go/v30 v30.8.1
vgo: finding github.com/stripe/stripe-go/v30 v30.8.1
vgo: downloading github.com/stripe/stripe-go/v30 v30.8.1
vgo: resolving import "github.com/stripe/stripe-go/form"
vgo: finding github.com/stripe/stripe-go (latest)
vgo: adding github.com/stripe/stripe-go v1.0.3
vgo: import "github.com/brandur/stripe-go-vgo" ->
        import "github.com/stripe/stripe-go/v30": found in both github.com/stripe/stripe-go v1.0.3 and github.com/stripe/stripe-go/v30 v30.8.1
```

I think what's happening is that it correctly sees the `v30.*` tag and resolves
that, but then it starts to descend into subpackages, and eventually finds one
that references the top-level package (almost all of them do, but take
`account/` as an example). The import doesn't use the `stripe/stripe-go/v30`
suffix, and therefore vgo tries to resolve a difference package version (once
again falling back to `v1.0.3`), and produces a conflict.

I think the fix is to make sure all imports end in `/v30`, but this is very
problematic from two angles:

1. It breaks out compatibility with non-vgo installations.
2. It forces us to update the import path on dozens of subpackages every time
   we do a new major release, which is very bad.

For now, I'm going to revert this. Hopefully some progress on vgo in the future
makes this a little easier.

README.md

@@ -36,21 +36,6 @@ changelog](CHANGELOG) and [API changelog][api-changelog].
 go get github.com/stripe/stripe-go
 ```
 
-Then import the package from your Go code:
-
-``` go
-import "github.com/stripe/stripe-go"
-```
-
-### vgo
-
-If using [vgo], don't `go get` it manually, but do use an `import` statement
-that includes the latest major version:
-
-``` go
-import "github.com/stripe/stripe-go/v30"
-```
-
 ## Documentation
 
 For a comprehensive list of examples, check out the [API
@@ -328,7 +313,6 @@ pull request][pulls].
 [package-management]: https://code.google.com/p/go-wiki/wiki/PackageManagementTools
 [pulls]: https://github.com/stripe/stripe-go/pulls
 [stripe]: https://stripe.com
-[vgo]: https://github.com/golang/vgo
 
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