exec shell: remove shell config or option

The option is never worked and can't properly work because almost all containers anyway using Ash/Dash from /bin/sh

config/param.go

@@ -12,11 +12,6 @@ var defaultParams = []*Param{
 		Val:   "state",
 		Label: "Container Sort Field",
 	},
-	&Param{
-		Key:   "shell",
-		Val:   "sh",
-		Label: "Shell",
-	},
 	&Param{
 		Key:   "columns",
 		Val:   "status,name,id,cpu,mem,net,io,pids",