removing ssz-only flag ( reverting feature) and fix accept header middleware (#15433)

* removing ssz-only flag

* gaz

* reverting other uses of sszonly

* gaz

* adding kasey and radek's suggestions

* update changelog

* adding test

* radek advice with new headers and tests

* adding logs and fixing comments

* adding logs and fixing comments

* gaz

* Update validator/client/beacon-api/rest_handler_client.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/apiutil/header.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* Update api/apiutil/header.go

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

* radek's comments

* adding another failing case based on radek's suggestion

* another unit test

---------

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

config/features/config.go

@@ -51,7 +51,6 @@ type Flags struct {
 	EnableExperimentalAttestationPool   bool // EnableExperimentalAttestationPool enables an experimental attestation pool design.
 	DisableDutiesV2                     bool // DisableDutiesV2 sets validator client to use the get Duties endpoint
 	EnableWeb                           bool // EnableWeb enables the webui on the validator client
-	SSZOnly                             bool // SSZOnly forces the validator client to use SSZ for communication with the beacon node when REST mode is enabled (useful for debugging)
 	// Logging related toggles.
 	DisableGRPCConnectionLogs bool // Disables logging when a new grpc client has connected.
 	EnableFullSSZDataLogging  bool // Enables logging for full ssz data on rejected gossip messages
@@ -340,10 +339,6 @@ func ConfigureValidator(ctx *cli.Context) error {
 		logEnabled(EnableWebFlag)
 		cfg.EnableWeb = true
 	}
-	if ctx.Bool(SSZOnly.Name) {
-		logEnabled(SSZOnly)
-		cfg.SSZOnly = true
-	}
 
 	cfg.KeystoreImportDebounceInterval = ctx.Duration(dynamicKeyReloadDebounceInterval.Name)
 	Init(cfg)