Need to specify variables when uninstalling a plugin

# Bug Report

## Problem

### What is expected to happen?

I can uninstall the plugin without having to specify variables.

### What does actually happen?

I have this plugins that requires 2 variables to install:

```
E:\Projects\throwaway\cordovaPlatformVerificationPlugin (helloworld@1.0.0)
λ cordova plugin add E:\Projects\Cordova\cordova-paramedic\spec\platform-verification-plugin\tests --variable PLATFORM=foo --variable VERSION=bar Plugin "platform-verification-plugin.tests" already installed on browser.
Adding platform-verification-plugin.tests to package.json
```

defined via `plugin.xml`:
```


```

But when I try to uninstall it, I also have to supply the variables:
```
E:\Projects\throwaway\cordovaPlatformVerificationPlugin (helloworld@1.0.0)
λ cordova plugin rm platform-verification-plugin.tests
Variable(s) missing (use: --variable PLATFORM=value --variable VERSION=value).
```

With the variable supplied I can uninstall the plugin:

```
E:\Projects\throwaway\cordovaPlatformVerificationPlugin (helloworld@1.0.0)
λ cordova plugin rm platform-verification-plugin.tests --variable PLATFORM=foo --variable VERSION=bar
Uninstalling 1 dependent plugins.
Uninstalling cordova-plugin-device from browser
js-module uninstall called : plugins\cordova-plugin-device\www\device.js
js-module uninstall called : plugins\cordova-plugin-device\src\browser\DeviceProxy.js
Uninstalling platform-verification-plugin.tests from browser
js-module uninstall called : plugins\platform-verification-plugin.tests\tests.js
Removing "platform-verification-plugin.tests"
Removing platform-verification-plugin.tests from package.json
```

## Information

### Command or Code
`cordova plugin rm platform-verification-plugin.tests`

### Version information

```
λ cordova info
cordova-lib@9.0.1 with:
cordova-common@3.1.0
cordova-create@2.0.0
cordova-fetch@2.0.1
cordova-serve@3.0.0

Environment:
OS: win32
Node: v12.3.1
npm: 6.9.0
```

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start by reproducing `cordova plugin rm platform-verification-plugin.tests` with the plugin.xml preferences PLATFORM and VERSION, then trace the plugin-removal command. Done means the plugin uninstalls without requiring those variables, while installation still supports them.

Written by the indexing model from the issue text.