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.

Adding a proper delete for unchanged files would be possible
though.

For now index will contain entries for files that don't exist,
but they won't be in the search results as 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>