Fix #1876397 [Apply button doesn't activate when removing lines from Search & Replace](https://bugs.launchpad.net/calibre/+bug/1876397)

src/calibre/gui2/convert/search_and_replace.py

@@ -103,6 +103,7 @@ def sr_remove_clicked(self):
             self.search_replace.setCurrentCell(row if row < self.search_replace.rowCount() else row-1, 0)
             self.sr_search.clear()
             self.sr_replace.clear()
+            self.changed_signal.emit()
 
     def sr_load_clicked(self):
         files = choose_files(self, 'sr_saved_patterns',