store/fields: Use one MaxTermLength everywhere

One should be enough!

lib/message/mu-fields.cc

@@ -22,9 +22,6 @@
 
 using namespace Mu;
 
-// Xapian does not like terms much longer than this
-constexpr auto MaxTermLength = 240;
-
 std::string
 Field::xapian_term(const std::string& s) const
 {

lib/message/mu-fields.hh

@@ -30,6 +30,10 @@
 
 namespace Mu {
 
+// Xapian does not like terms much longer than this
+constexpr auto MaxTermLength = 240;
+// http://article.gmane.org/gmane.comp.search.xapian.general/3656 */
+
 struct Field {
 	/**
 	 * Field Ids.

lib/mu-server.cc

@@ -572,7 +572,7 @@ Server::Private::contacts_handler(const Parameters& params)
 static std::vector<Store::Id>
 docids_for_msgid(const Store& store, const std::string& msgid, size_t max = 100)
 {
-	if (msgid.size() > Store::MaxTermLength) {
+	if (msgid.size() > MaxTermLength) {
 		throw Error(Error::Code::InvalidArgument,
 			    "invalid message-id '%s'", msgid.c_str());
 	}

lib/mu-store.hh

@@ -43,8 +43,6 @@ public:
 	using Id                      = Xapian::docid; /**< Id for a message in the store */
 	static constexpr Id InvalidId = 0;             /**< Invalid  store id */
 
-	static constexpr size_t MaxTermLength = 240; /**< Maximum length of a term,
-	  http://article.gmane.org/gmane.comp.search.xapian.general/3656 */
 	/**
 	 * Construct a store for an existing document database
 	 *