`ionic capacitor build` produces source maps

**Description:**

Running the `ionic capacitor build` command produces source maps in `build/static` folder. I __do not__ specify the `--source-map` option. With or without the `--prod` option.
According to the [docs](https://ionicframework.com/docs/cli/commands/capacitor-build#advanced-options), source maps must only be produced when `--source-map` is specified.
As you can see from the --verbose output, source maps are being generated by default.

**Steps to Reproduce:**

See Description

**Output:**

See attached file
[log.txt](https://github.com/ionic-team/ionic-cli/files/6888305/log.txt)

**My `ionic info`:**

```
[WARN] Error loading @capacitor/ios package.json: Error: Cannot find module '@capacitor/ios/package'

Require stack:
- /usr/local/lib/node_modules/@ionic/cli/lib/project/index.js
- /usr/local/lib/node_modules/@ionic/cli/lib/index.js
- /usr/local/lib/node_modules/@ionic/cli/index.js
- /usr/local/lib/node_modules/@ionic/cli/bin/ionic

Ionic:

Ionic CLI : 6.16.1 (/usr/local/lib/node_modules/@ionic/cli)
Ionic Framework : @ionic/react 5.6.12

Capacitor:

Capacitor CLI : 3.1.2
@capacitor/android : 3.1.2
@capacitor/core : 3.1.2
@capacitor/ios : not installed

Utility:

cordova-res : not installed globally
native-run : 1.4.0

System:

NodeJS : v16.1.0 (/usr/local/bin/node)
npm : 7.11.2
OS : macOS Mojave
```

Start by reproducing `ionic capacitor build` with and without `--source-map`, using the attached `log.txt` and both with and without `--prod`. Trace the `ionic capacitor build` entry point to where `build/static` output is configured; done means source maps are absent by default and present when `--source-map` is specified.

由索引模型根據 Issue 內容生成。