Remove api_diff_check CI action due to issue with module hashes (#1164)

Removes the api_diff_check GitHub CI action since it was failing due to
the SDK's inconsistent go.sum and go.mod files throughout its various
modules. Until these can be resolved, and gorelease updated to ignore
go.sum files this action needs to be removed.

Makefile

@@ -229,6 +229,21 @@ cachedep-modules-%:
 		&& go run . -p $(subst _,/,$(subst cachedep-modules-,,$@)) ${EACHMODULE_FLAGS} \
 		"go mod download"
 
+api-diff-modules-%:
+	@# Command that uses the pattern to define the root path that the
+	@# module testing will start from. Strips off the "api-diff-modules-" and
+	@# replaces all "_" with "/".
+	@#
+	@# Requires golang.org/x/exp/cmd/gorelease to be available in the GOPATH.
+	@#
+	@# e.g. api-diff-modules-internal_protocoltest
+	cd ./internal/repotools/cmd/eachmodule \
+		&& go run . -p $(subst _,/,$(subst api-diff-modules-,,$@)) \
+			-fail-fast=true \
+			-c 1 \
+			-skip="internal/repotools" \
+			"$$(go env GOPATH)/bin/gorelease"
+
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 # CI Testing #
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