metainfo: Drop categories from metainfo.xml

**Categories and keywords**
> If there’s a launchable defined for a desktop application, categories and keywords are pulled from the desktop file. Defining them separately in the Metainfo file will override the contents of the desktop file.

Source: https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines#categories-and-keywords

Signed-off-by: Sabri Ünal <yakushabb@gmail.com>
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/812>

data/org.gnome.Calls.metainfo.xml.in

@@ -55,10 +55,6 @@
         <kudo>Notifications</kudo>
     </kudos>
 
-    <categories>
-        <category>Audio</category>
-    </categories>
-
     <provides>
         <mediatype>x-scheme-handler/tel</mediatype>
         <mediatype>x-scheme-handler/sip</mediatype>