Travis: remove obsolete Maven settings

.travis/settings.xml

-<settings>
-  <servers>
-    <server>
-      <id>imagej.releases</id>
-      <username>travis</username>
-      <password>${env.MAVEN_PASS}</password>
-    </server>
-    <server>
-      <id>imagej.snapshots</id>
-      <username>travis</username>
-      <password>${env.MAVEN_PASS}</password>
-    </server>
-  </servers>
-</settings>