Revert setting unblockSelectorSignal SDK flag by default (#2070)

Author: yuandrew

internal/internal_flags.go

@@ -2,6 +2,7 @@ package internal
 
 import (
 	"math"
+	"os"
 
 	"go.temporal.io/api/workflowservice/v1"
 )
@@ -33,7 +34,7 @@ const (
 
 // unblockSelectorSignal exists to allow us to configure the default behavior of
 // SDKFlagBlockedSelectorSignalReceive. This is primarily useful with tests.
-var unblockSelectorSignal = true
+var unblockSelectorSignal = os.Getenv("UNBLOCK_SIGNAL_SELECTOR") != ""
 
 func sdkFlagFromUint(value uint32) sdkFlag {
 	switch value {