Remove UNRESERVED as a reservation type

This is a follow-up to Lai's refactoring of the get reservation types
code to return a set rather than a single type. Since we're always
returning a set now, the more natural way to represent a label that is
not reserved is to return an empty set rather than a set containing
UNRESERVED.

Also fixes some minor style issues I ran across regarding static
importing and test method naming that I ran across (no logic
implications).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151132116

javatests/google/registry/export/ExportReservedTermsActionTest.java

@@ -74,8 +74,7 @@ public class ExportReservedTermsActionTest {
     ReservedList rl = persistReservedList(
         "tld-reserved",
         "lol,FULLY_BLOCKED",
-        "cat,FULLY_BLOCKED",
-        "jimmy,UNRESERVED");
+        "cat,FULLY_BLOCKED");
     createTld("tld");
     persistResource(Registry.get("tld").asBuilder()
         .setReservedLists(rl)