Remove custom font size for IM URL

As best as I can tell, the designs call for the IM URL to be the same size as
other subheadings. The font size in Zeplin doesn't match what is currently used
in Settings, so this likely caused confusion.

res/css/views/settings/_SetIntegrationManager.scss

@@ -30,5 +30,4 @@ limitations under the License.
 .mx_SetIntegrationManager > .mx_SettingsTab_heading > .mx_SettingsTab_subheading {
     display: inline-block;
     padding-left: 5px;
-    font-size: 14px;
 }