README: remove pkgx installation instructions

See #659 (comment).

Author: FiloSottile

README.md

@@ -145,12 +145,6 @@ $ age --decrypt -i key.txt data.tar.gz.age > data.tar.gz
             <code>scoop bucket add extras && scoop install age</code>
         </td>
     </tr>
-    <tr>
-        <td>pkgx</td>
-        <td>
-            <code>pkgx install age</code>
-        </td>
-    </tr>
 </table>
 
 On Windows, Linux, macOS, and FreeBSD you can use the pre-built binaries.