Chomp trailing _URL when removing Heroku Postgresql databases

Make `HEROKU_POSTGRESQL_BRONZE_URL` valid instead of insisting on
`HEROKU_POSTGRESQL_BRONZE`

Output before:

```
$ heroku addons:remove HEROKU_POSTGRESQL_BRONZE_URL -a test-thing 

 !    WARNING: Destructive Action
 !    This command will affect the app: test-thing
 !    To proceed, type "test-thing" or re-run this command with --confirm test-thing

> test-thing
Removing HEROKU_POSTGRESQL_BRONZE_URL on test-thing... failed
 !    AddonPlan not found.
```

After (the `bin/` is from my local checkout):

```
$ bin/heroku addons:remove HEROKU_POSTGRESQL_BRONZE_URL -a test-thing                                                          

 !    WARNING: Destructive Action
 !    This command will affect the app: test-thing
 !    To proceed, type "test-thing" or re-run this command with --confirm test-thing

> test-thing
Removing HEROKU_POSTGRESQL_BRONZE on test-thing... done, v119 ($50/mo)
```