font-patcher: Correct essential ligatures

[why]
The essential glyphs are partially based on which substitution rules
the font has.

And the config.json configuration is able to remove some substitution
rules.

So we must first possibly remove rules and then examine them, not the
other way around, because now we detect 'essential glyphs' for rules
that we later remove.

[how]
Correct order of operation.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -338,9 +338,9 @@ class font_patcher:
     def patch(self, font):
         self.sourceFont = font
         self.setup_version()
-        self.get_essential_references()
         self.assert_monospace()
         self.remove_ligatures()
+        self.get_essential_references()
         self.get_sourcefont_dimensions()
         self.setup_patch_set()
         self.improve_line_dimensions()