Delete the build-agent module and the builder program: the build module builds, the checker checks (issue 1851, C2a)

Makefile

@@ -46,17 +46,6 @@ image:
 	@echo
 	@docker image inspect $(IMAGE) --format 'built {{.RepoTags}}  {{.Size}} bytes'
 
-# The builder ships as an image too, because it is a module the mesh assigns rather than a program
-# somebody starts on a machine by hand.
-BUILDER_IMAGE ?= mesh-builder:$(VERSION)
-BUILDER_DEV_TAG ?= mesh-builder:development
-
-builder-image:
-	@test -n "$(GO_BASE)" || { echo "no GO_BASE; pass GO_BASE=<image>"; exit 1; }
-	docker build --build-arg GO_BASE=$(GO_BASE) -f cmd/mesh-builder/Dockerfile -t $(BUILDER_IMAGE) -t $(BUILDER_DEV_TAG) .
-	@echo
-	@docker image inspect $(BUILDER_IMAGE) --format 'built {{.RepoTags}}  {{.Size}} bytes'
-
 # The provisioner ships as an image too, because it is the thing that makes a sealed credential
 # true on a machine -- and the mesh cannot, having discarded the plaintext.
 PROVISIONER_IMAGE ?= mesh-provision-postgres:$(VERSION)