ci: remove unused CircleCI Makefile (#12828)

This Makefile was used to generate the full config.yml from smaller
sub-files, but this is not done anymore.

.circleci/Makefile

@@ -1,37 +0,0 @@
-# Set SHELL to 'strict mode' without using .SHELLFLAGS for max compatibility.
-# See https://fieldnotes.tech/how-to-shell-for-compatible-makefiles/
-SHELL := /usr/bin/env bash -euo pipefail -c
-
-CIRCLECI := circleci --skip-update-check
-
-# Set up some documentation/help message variables.
-# We do not attempt to install the CircleCI CLI from this Makefile.
-CCI_INSTALL_LINK := https://circleci.com/docs/2.0/local-cli/\#installation
-CCI_INSTALL_MSG := Please install CircleCI CLI. See $(CCI_INSTALL_LINK)
-CCI_VERSION := $(shell $(CIRCLECI) version 2> /dev/null)
-ifeq ($(CCI_VERSION),)
-# Attempting to use the CLI fails with installation instructions.
-CIRCLECI := echo '$(CCI_INSTALL_MSG)'; exit 1; \#
-endif
-
-default: help
-
-help:
-	@echo "Usage:"
-	@echo "  make ci-verify: validate that config.yml for correctness"
-	@echo
-	@echo "Diagnostics:"
-	@[ -z "$(CCI_VERSION)" ] || echo "  circleci-cli version $(CCI_VERSION)"
-	@[ -n "$(CCI_VERSION)" ] || echo "  $(CCI_INSTALL_MSG)"
-
-.PHONY: ci-verify
-ci-verify:
-	@$(CIRCLECI) config validate config.yml
-
-MKFILE_PATH := $(lastword $(filter %Makefile,$(MAKEFILE_LIST)))
-CURRENT_DIR := $(dir $(realpath $(MKFILE_PATH)))
-CURRENT_DIR := $(CURRENT_DIR:/=)
-
-ifneq ($(wildcard $(CURRENT_DIR)/*.mk),)
-include $(CURRENT_DIR)/*.mk
-endif