patch 8.2.4808: unused item in engine struct

Problem:    Unused item in engine struct.
Solution:   Remove "expr".  Add comment with tags.

src/regexp.h

@@ -166,11 +166,15 @@ typedef struct
 
 struct regengine
 {
+    // bt_regcomp or nfa_regcomp
     regprog_T	*(*regcomp)(char_u*, int);
+    // bt_regfree or nfa_regfree
     void	(*regfree)(regprog_T *);
+    // bt_regexec_nl or nfa_regexec_nl
     int		(*regexec_nl)(regmatch_T *, char_u *, colnr_T, int);
+    // bt_regexec_mult or nfa_regexec_mult
     long	(*regexec_multi)(regmmatch_T *, win_T *, buf_T *, linenr_T, colnr_T, proftime_T *, int *);
-    char_u	*expr;
+    //char_u	*expr;
 };
 
 #endif	// _REGEXP_H

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4808,
 /**/
     4807,
 /**/