Remove FEATURE_INCREASED_KEEPALIVE in production

[AntoineAugusti]

We added an environment variable on the command line to be able to test/rollback a feature in production. It's working as expected now so we removed the code check for this env variable in #76 cds-snc/notification-admin#856.

We need to remove this env variable in production as it's not part of the manifests

kubectl set env deployment/api FEATURE_INCREASED_KEEPALIVE-
kubectl set env deployment/admin FEATURE_INCREASED_KEEPALIVE-

[AntoineAugusti]

This has been done yesterday in production after chatting with @jimleroyer