chore: remove frappe from requirements.txt

Due to recent changes in pip dependency resolver, in some random cases
pip thinks frappe is not installed and tries to fetch it from pypi,
which results in errors like #25496

Until a better solution is available, frappe should not be part
of requirements.txt

requirements.txt

@@ -1,4 +1,4 @@
-frappe
+# frappe   # https://github.com/frappe/frappe is installed during bench-init
 gocardless-pro~=1.22.0
 googlemaps  # used in ERPNext, but dependency is defined in Frappe
 pandas~=1.1.5