Disable source maps for Flipper release builds

Summary:
Disabled source maps for Flipper release builds.
1) I found that source maps was not only written to the "bundle.map" file, but also inlined into "bundle.js" in base64 format which hugely affected the total bundle size.
2) In addition to the size, these inlined source maps were also one of 2 causes that Flipper package size was not deterministic and changed so significantly in every diff even when Flipper was not touched at all.

Changelog: Disabled source maps in Flipper release builds thus reducing size by ~20%.

Reviewed By: mweststrate

Differential Revision: D22158898

fbshipit-source-id: b35cfba80553d01aec256983c3ba772ed0534d8f

desktop/scripts/build-utils.ts

@@ -97,7 +97,7 @@ async function compile(
       dev,
       minify: !dev,
       resetCache: !dev,
-      sourceMap: true,
+      sourceMap: dev,
       entry,
       out: path.join(buildFolder, 'bundle.js'),
     },
@@ -174,7 +174,7 @@ export async function compileMain() {
       out,
       dev,
       minify: !dev,
-      sourceMap: true,
+      sourceMap: dev,
       resetCache: !dev,
     });
     console.log('✅  Compiled main bundle.');