Remove obsolete highlight_group keys from configuration

They are no no-op for function segments, so don’t confuse users

powerline/config_files/themes/shell/default.json

@@ -35,8 +35,7 @@
 		"right": [
 			{
 				"module": "powerline.segments.shell",
-				"name": "last_pipe_status",
-				"highlight_group": "exit_fail"
+				"name": "last_pipe_status"
 			},
 			{
 				"name": "branch"

powerline/config_files/themes/shell/default_leftonly.json

@@ -36,7 +36,6 @@
 			},
 			{
 				"name": "last_status",
-				"highlight_group": ["exit_fail"],
 				"module": "powerline.segments.shell"
 			}
 		]

powerline/config_files/themes/vim/default.json

@@ -100,7 +100,6 @@
 				"priority": 30,
 				"before": ":",
 				"width": 3,
-				"highlight_group": ["col_current"],
 				"align": "l"
 			}
 		]