Remove references to os for Edge Runtime compatibility (#115)

## Problem

PR #110 resolved the problem with `fs` imports that was happening in the
Edge Runtime, but feedback on a dev build shows that `os` imports are
still an issue even though the `@edge-runtime/jest-environment` test
build is passing.

This relates to:
- #108 

## Solution

For now, the easiest thing is to just remove references to `os`. The
information it adds to the user-agent header is nice to have, but not
essential.

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)

src/utils/user-agent.ts

@@ -1,4 +1,3 @@
-import { getOSInfo } from './os';
 import { isEdge } from './environment';
 import * as packageInfo from '../version.json';
 
@@ -11,12 +10,7 @@ export const buildUserAgent = (isLegacy: boolean) => {
     'lang=typescript',
   ];
 
-  if (!isEdge()) {
-    const osInfo = getOSInfo();
-    if (osInfo) {
-      userAgentParts.push(osInfo);
-    }
-  } else {
+  if (isEdge()) {
     userAgentParts.push('Edge Runtime');
   }