Remove deprecated CLI and env vars

[chrisdoherty4]

0.7.0 deprecated a set of env variables in favor of standarized ones. They can be removed in 0.8.0.

From the help menu:

Deprecated CLI flags
  Deprecated   Current
  --http_port  --http-port
  --tls_cert   --grpc-tls-cert
  --tls_key    --grpc-tls-key
  --use_tls    --grpc-use-tls

Deprecated environment variables
  Deprecated          Current
  CUSTOM_ENDPOINTS    HEGEL_HTTP_CUSTOM_ENDPOINTS
  DATA_MODEL_VERSION  HEGEL_DATA_MODEL
  GRPC_PORT           HEGEL_GRPC_PORT
  HEGEL_TLS_CERT      HEGEL_GRPC_TLS_CERT
  HEGEL_TLS_KEY       HEGEL_GRPC_TLS_KEY
  HEGEL_USE_TLS       HEGEL_GRPC_USE_TLS
  TRUSTED_PROXIES     HEGEL_TRUSTED_PROXIES