Use shorter arrow functions where possible

For arrow functions that are both simple and short, we can avoid using explicit `return` to shorten them even further without hurting readability.

For the `gulp mozcentral` build-target this reduces the overall size of the output by just under 1 kilo-byte (which isn't a lot but still can't hurt).

test/unit/cff_parser_spec.js

@@ -377,9 +377,7 @@ describe("CFFCompiler", function () {
     bytes = new Uint8Array(bytes);
     return new CFFParser(
       {
-        getBytes: () => {
-          return bytes;
-        },
+        getBytes: () => bytes,
       },
       {},
       SEAC_ANALYSIS_ENABLED