refactor: remove redundant struct names

A struct can be anonymous if only its typedef is used.

src/nvim/cmdexpand_defs.h

@@ -15,7 +15,7 @@ typedef enum {
 enum { EXPAND_BUF_LEN = 256, };
 
 /// used for completion on the command line
-typedef struct expand {
+typedef struct {
   char *xp_pattern;             ///< start of item to expand, guaranteed
                                 ///< to be part of xp_line
   int xp_context;               ///< type of expansion