Remove unwanted semicolon in assets files

[nlssn]

Fixes #2120

Changes proposed in this pull request:
Removes an unnecessary / unwanted semicolon that is generated in /assets/forum-<hash>.js and /assets/admin-<hash>.js.

As mentioned by @clarkwinkelmann the problem is found here:
https://github.com/flarum/core/blob/d492579638fb52dafbfe65f1f36a95eb6047f7f3/src/Frontend/Compiler/JsCompiler.php#L72

I simply removed the extra semicolon. No errors showing.

Confirmed

- [ ] Frontend changes: tested on a local Flarum installation.
- [ ] Backend changes: tests are green (run composer test).

[clarkwinkelmann]

Note for the future: this PR actually changed more than I expected, but it's not creating any issue.

What was before

var module={};

modules.export = /* extension code*/;
;

flarum.extensions['extensionName']=module.exports;

is now

var module={}

modules.export = /* extension code*/;

flarum.extensions['extensionName']=module.exports

The semicolumn after module=[} and =module.exports was never part of the Frontend extender, instead it was an artifact because Flarum was adding a sourcemap for each individual source. Which means one extension was actually 3 sources, each with a sourcemap being replaced and with a semicolumn added at the end.