Remove unnecessary (char *) casts [string]

Remove (char *) casts around string functions where the arguments or
result already have the right type and the cast is useless (or worse,
potentially casts away a qualifier, but this doesn't appear to be the
case here).

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/fd1fcedb-3492-4fc8-9e3e-74b97f2db6c7%40eisentraut.org

doc/src/sgml/gist.sgml

@@ -1027,7 +1027,7 @@ fill_my_string_relopt(const char *value, void *ptr)
     int     len = strlen(tmp);
 
     if (ptr)
-        strcpy((char *) ptr, tmp);
+        strcpy(ptr, tmp);
 
     pfree(tmp);
     return len + 1;