fix(api): remove explicit dependency on prometheus-client

We no longer need this pinning (the original reason, which was due to using a
private interface, is no longer a problem).

Related: c350aef).

api/requirements.txt

@@ -13,7 +13,6 @@ dnspython~=2.4.2
 httpretty~=1.0.5  # 1.1 breaks tests. Does not run in production, so stick to it.
 pyotp~=2.9.0
 psycopg~=3.1.12
-prometheus-client~=0.17.1  # added to control django-prometheus' dependency version
 psl-dns~=1.1.0
 pylibmc~=1.6.3
 pyyaml~=6.0.1