Introduce method to unset the query string

This resolves TYPO3-Solr#4135

Classes/Domain/Search/SearchRequest.php

@@ -444,6 +444,17 @@ public function setRawQueryString(string $rawQueryString = ''): SearchRequest
         return $this;
     }
 
+    /**
+     * Unsets the query string.
+     */
+    public function removeQueryString(): SearchRequest
+    {
+        $this->stateChanged = true;
+        $path = $this->prefixWithNamespace('q');
+        $this->argumentsAccessor->reset($path);
+        return $this;
+    }
+
     /**
      * Returns the passed rawQueryString.
      */