LibIMAP: Remove unused Variant option

Userland/Libraries/LibIMAP/Objects.cpp

@@ -138,7 +138,6 @@ String serialize_astring(StringView string)
 String SearchKey::serialize() const
 {
     return data.visit(
-        [&](Empty const&) { VERIFY_NOT_REACHED(); return String("The compiler complains if you remove this."); },
         [&](All const&) { return String("ALL"); },
         [&](Answered const&) { return String("ANSWERED"); },
         [&](Bcc const& x) { return String::formatted("BCC {}", serialize_astring(x.bcc)); },

Userland/Libraries/LibIMAP/Objects.h

@@ -460,7 +460,7 @@ public:
     struct Unseen { };
     // clang-format on
 
-    Variant<Empty, All, Answered, Bcc, Cc, Deleted, Draft, From, Header, Keyword,
+    Variant<All, Answered, Bcc, Cc, Deleted, Draft, From, Header, Keyword,
         Larger, New, Not, Old, On, Or, Recent, SearchKeys, Seen, SentBefore, SentOn,
         SentSince, SequenceSet, Since, Smaller, Subject, Text, To, UID, Unanswered,
         Undeleted, Undraft, Unkeyword, Unseen>