Simplify the method of removing broken arch packages from the main list

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -2499,7 +2499,7 @@ class Britney:
         archpackages = {}
         for a in self.options.break_arches.split():
             archpackages[a] = [p for p in normpackages if p.endswith("/" + a) or p.endswith("/" + a + "_tpu")]
-            normpackages = [p for p in normpackages if not (p.endswith("/" + a) or p.endswith("/" + a + "_tpu"))]
+            normpackages = [p for p in normpackages if p not in archpackages[a]]
         self.upgrade_me = normpackages
         self.output_write("info: main run\n")
         self.do_all()