Remove the vaadin-snapshots repo (#12420)

uitest/pom.xml

@@ -34,16 +34,6 @@
                 <enabled>true</enabled>
             </snapshots>
         </repository>
-        <repository>
-            <id>vaadin-snapshots</id>
-            <url>http://oss.sonatype.org/content/repositories/vaadin-snapshots</url>
-            <releases>
-                <enabled>false</enabled>
-            </releases>
-            <snapshots>
-                <enabled>true</enabled>
-            </snapshots>
-        </repository>
     </repositories>
 
     <dependencies>