Remove Image watcher

[nakabonne]

What this PR does / why we need it:
Remove all about Image watcher as we settled on supporting Event watcher instead of it.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 33.47%. This pull request decreases coverage by -0.03%.

File	Function	Base	Head	Diff
pkg/config/config.go	Config.init	96.00%	95.65%	-0.35%
pkg/config/piped.go	PipedSpec.Validate	55.56%	56.25%	+0.69%
pkg/config/piped.go	PipedEventWatcher.Validate	0.00%	100.00%	+100.00%
pkg/app/piped/logpersister/stagelogpersister.go	stageLogPersister.flushFromLastCheckpoint	50.00%	61.11%	+11.11%
pkg/app/piped/imageprovider/ecr/ecr.go	WithRegistryID	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	WithCredentialsFile	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	WithProfile	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	WithLogger	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	NewECR	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	ECR.Name	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	ECR.Type	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	ECR.ParseImage	0.00%	--	+-0.00%
pkg/app/piped/imageprovider/ecr/ecr.go	ECR.GetLatestImage	0.00%	--	+-0.00%
pkg/config/image_watcher.go	LoadImageWatcher	0.00%	--	+-0.00%
pkg/config/image_watcher.go	filterImageWatcherFiles	100.00%	--	-100.00%
pkg/config/image_watcher.go	ImageWatcherSpec.Validate	100.00%	--	-100.00%
pkg/config/piped.go	PipedImageProvider.UnmarshalJSON	88.89%	--	-88.89%
pkg/config/piped.go	PipedImageWatcher.Validate	100.00%	--	-100.00%
pkg/model/image_name.go	ImageName.String	100.00%	--	-100.00%
pkg/model/image_name.go	ImageName.Name	0.00%	--	+-0.00%
pkg/model/image_name.go	ImageRef.String	100.00%	--	-100.00%
pkg/model/imageprovider.go	ImageProviderType.String	0.00%	--	+-0.00%

[nghialv]

Good feeling. 👍
/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.