Postproccess tries to move .map files on --prod builds

[j3gb3rt]

Short description of the problem:

Postprocess is checking to move source source maps on production build.
It is failing because on production these are not generate.
Fails with a file does not exist error

What behavior are you expecting?

Map should not try to be moved on production builds as they are not generated

Steps to reproduce:

1. Build a cordova release using

ionic cordova build android --prod --release

or

ionic cordova build ios --prod --release

or

npm run ionic-app-scripts build --prod

Which @ionic/app-scripts version are you using?
2.1.4

Other information:
cli packages:

@ionic/cli-utils  : 1.9.2
ionic (Ionic CLI) : 3.9.2

global packages:

Cordova CLI : 7.0.1
Gulp CLI    : not installed globally

local packages:

@ionic/app-scripts : 2.1.4
Cordova Platforms  : android 6.2.3 ios 4.4.0
Ionic Framework    : ionic-angular 3.6.0

System:

Android SDK Tools : 26.0.2
ios-deploy        : 1.9.1
ios-sim           : 6.0.0
Node              : v6.11.2
npm               : 3.10.10
OS                : macOS Sierra
Xcode             : Xcode 8.3.2 Build version 8E2002