Cleanup: Remove an unused variable

log_write_buf(): Remove the unused variable write_header.
The dependent code was removed in
commit 0c2365c4.
That was dead or unnecessary code at least ever since
commit 9ef2d29f
removed the support for innodb_log_files_in_group>1.

storage/innobase/log/log0log.cc

@@ -864,7 +864,6 @@ log_write_buf(
 					header */
 {
 	ulint		write_len;
-	bool		write_header	= new_data_offset == 0;
 	lsn_t		next_offset;
 	ulint		i;
 
@@ -929,9 +928,6 @@ log_write_buf(
 		start_lsn += write_len;
 		len -= write_len;
 		buf += write_len;
-
-		write_header = true;
-
 		goto loop;
 	}
 }