remove unused ENVOY_IMAGE Makefile variable

Signed-off-by: Steve Kriss <stephen.kriss@gmail.com>

Makefile

@@ -6,7 +6,6 @@ IMAGE := $(REGISTRY)/$(PROJECT)
 SRCDIRS := ./cmd ./internal ./apis
 LOCAL_BOOTSTRAP_CONFIG = localenvoyconfig.yaml
 SECURE_LOCAL_BOOTSTRAP_CONFIG = securelocalenvoyconfig.yaml
-ENVOY_IMAGE = docker.io/envoyproxy/envoy:v1.31.0
 GATEWAY_API_VERSION ?= $(shell grep "sigs.k8s.io/gateway-api" go.mod | awk '{print $$2}')
 
 # Used to supply a local Envoy docker container an IP to connect to that is running