Remove `alluxio.user.hostname` from alluxio/conf/RemovedKey.java

[LuQQiu]

alluxio.user.hostname is not deprecated and is still used
remove the line

      put("alluxio.user.hostname",
          replacedSince(V2_0_0, PropertyKey.LOCALITY_TIER_NODE.getName()));

[Xaprice]

/assign @Xaprice