engine: Remove in-tree unicodesn stemmer

Now that search is using libstemmer, the in-tree stemmer is no longer
needed and can be removed.

sql/version-011.sql

@@ -1,5 +1,16 @@
 --
--- Dummy database upgrade to add MessageSearchTable, whose parameters depend on
--- things we need at run-time.  See src/engine/imap-db/imap-db-database.vala in
--- post_upgrade() for the code that runs the upgrade.
+-- Create MessageSearchTable
 --
+
+CREATE VIRTUAL TABLE MessageSearchTable USING fts4(
+    body,
+    attachment,
+    subject,
+    from_field,
+    receivers,
+    cc,
+    bcc,
+
+    tokenize=simple,
+    prefix="2,4,6,8,10",
+);