Delete obsolete mockery make target and doc ref

This was only required for the test mocks for the V1 AWS SDK tests,
which have been removed with the rewrite to V2.

Signed-off-by: Hidde Beydals <hello@hidde.co>

CONTRIBUTING.md

@@ -21,10 +21,6 @@ Mozilla welcomes contributions from everyone. Here are a few guidelines and inst
 * Unless it's particularly hard, changes that fix a bug should have a regression test to make sure that the bug is not introduced again.
 * New features and changes to existing features should be documented, and, if possible, tested.
 
-# Regenerating mocks
-
-If you encounter an error like `kms/mocks/KMSAPI.go:1607: cannot use (*KMSAPI)(nil) (type *KMSAPI) as type kmsiface.KMSAPI in assignment: *KMSAPI does not implement kmsiface.KMSAPI (missing ListResourceTags method)`, you need to regenerate mocks, probably because the interface was changed by a vendoring update. There is a make command to do this for you. Simply run `make mock`, and the new mocks will be automatically generated.
-
 # Communication
 
 If you need any help contributing to sops, several contributors are on the `#go` channel on [Mozilla's IRC server](https://wiki.mozilla.org/IRC).

Makefile

@@ -112,8 +112,4 @@ endif
 download-index:
 	bash make_download_page.sh
 
-mock:
-	go install github.com/vektra/mockery/.../
-	mockery -dir vendor/github.com/aws/aws-sdk-go/service/kms/kmsiface/ -name KMSAPI -output kms/mocks
-
-.PHONY: all test generate clean vendor functional-tests mock
+.PHONY: all test generate clean vendor functional-tests