Remove legacy guessing logic from app-shell

[wagnermaciel]

Currently _getServerModuleBundlePath of app-shell/index.ts uses a regular expression to try to find the main bundle file.

https://github.com/angular/angular-cli/blob/master/packages/angular_devkit/build_angular/src/app-shell/index.ts#L111

This is unnecessary since the main bundle will always be named main.js

@alan-agius4 I noticed this issue as a result of your comment that revealed this guessing logic is no longer needed

angular/universal#1357 (comment)

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