Take out inline styles on ThemeEditor.jsx

We had some leftover inline styles on the iframe for Theme Editor. These are
now in the CSS.

Test Plan:
- Load up New UI and go to Theme Editor
- the Preview area should still look the same (ie, reach full height of window)

fixes: CNVS-21841

Change-Id: I504fd1d065565fca6def634938951dc0bba73ca6
Reviewed-on: https://gerrit.instructure.com/58278
Reviewed-by: Jennifer Stern <jstern@instructure.com>
Product-Review: Jennifer Stern <jstern@instructure.com>
Tested-by: Jenkins
QA-Review: Robert Lamb <rlamb@instructure.com>

app/jsx/theme_editor/ThemeEditor.jsx

@@ -177,7 +177,7 @@ define([
                 </button>
               </div>
             </div>
-            <iframe src="/?editing_brand_config=1" style={{border: 'none', width: '100%', height: '100vh'}} />
+            <iframe src="/?editing_brand_config=1" />
           </div>
         </form>
         </div>