CI: flake8: remove ignore for W605

This was for 'invalid escape sequence' (even though the comment has W604
instead of W605). After removing it, flake8 runs through fine. We
probably fixed whatever invalid escape squence we had earlier while
doing f-string related refactoring.

Reviewed-by: Luca Weiss <luca@z3ntu.xyz>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20221028074119.3309-1-ollieparanoid@postmarketos.org%3E

.ci/flake8.sh

@@ -11,8 +11,7 @@ fi
 # E402: module import not on top of file, not possible for testcases
 # E722: do not use bare except
 # W504: line break occurred after a binary operator
-# W604: invalid escape sequence
-ign="E402,E722,W504,W605"
+ign="E402,E722,W504"
 
 set -x