Make sure to clear cookies on logout (#5727)

* Ensure to clear the cookies when logging out

It turns out that we failed to clear the cookies from the cookie JAR
when logging the user out. As a consequence, the cookie were retained
and it was possible to edit depictions as the previous user even without
logging in to the app (using the retained cookies).

Make sure we properly clear the cookies when we log the user out.

As an aside, the fact that the edit button shouldn't have been shown
is a different issue being tracked in #5726

* session: reuse removeAccount method for log out

The removeAccount method takes care of invoking the non-deprecated
API in applicable API levels. The logout method did not do such a
thing. Avoid redundancy, and reuse the removeAccount method for
logging out.

app/src/main/java/fr/free/nrw/commons/wikidata/cookies/CommonsCookieJar.kt

@@ -95,4 +95,9 @@ class CommonsCookieJar(private val cookieStorage: CommonsCookieStorage) : Cookie
 
     private fun Cookie.domainSpec(url: HttpUrl): String =
         domain.ifEmpty { url.toUri().getAuthority() }
+
+    fun clear() {
+        cookieStorage.clear()
+    }
+
 }