patch 9.0.2086: code cleanup for option callbacks needed

Problem:  code cleanup for option callbacks needed
Solution: remove flag os_doskip, it's not necessary, as we can check,
          whether an error message was returned

Remove unnecessary field os_doskip

Callback functions for boolean options set os_doskip immediately before
returning an error message, so os_doskip isn't actually needed.

closes: #13461

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/structs.h

@@ -4948,10 +4948,6 @@ typedef struct
 	char_u	*string;
     } os_newval;
 
-    // When set by the called function: Stop processing the option further.
-    // Currently only used for boolean options.
-    int		os_doskip;
-
     // Option value was checked to be safe, no need to set P_INSECURE
     // Used for the 'keymap', 'filetype' and 'syntax' options.
     int		os_value_checked;