shared: migrations: Add 3.sql: Drop potentailly harmful trigger

In [1] it's stated that "If the values "inserted" into the text
columns as part of a 'delete' command are not the same as those
currently stored within the table, the results may be unpredictable."

It's to be assumed only inserting ftsid is unpredictable. We
have no way for a proper delete because files are not immutable
or may have been deleted.

For now the index will contain entries for files that don't exist.
They won't appear in search results as they won't be joined
in the query.

[1] https://www.sqlite.org/fts5.html#the_delete_command

shared/migrations/3.sql

@@ -0,0 +1 @@
+DROP trigger content_ad;

shared/migrations/migrations.qrc

@@ -2,5 +2,6 @@
     <qresource prefix="/looqs-migrations">
         <file>1.sql</file>
         <file>2.sql</file>
+        <file>3.sql</file>
     </qresource>
 </RCC>