terminal: remove process.env from browser

[paul-marechal]

process.env is not available when in a browser environment. Webpack most
likely included a shim for it.

How to test

We should get the same default values on both master and this PR for the following preferences, and it should be independent from your SHELL environment variable.

• terminal.integrated.shell.windows defaults to C:\\Windows\\System32\\cmd.exe
• terminal.integrated.shell.osx defaults to /bin/bash
• terminal.integrated.shell.linux defaults to /bin/bash

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM!