MDEV-13310 Preparing an incremental backup twice can corrupt data

Remove .delta file after it was successfully applied

Author: vaintroub

extra/mariabackup/xtrabackup.cc

@@ -4974,8 +4974,11 @@ xtrabackup_apply_delta(
 
 if (incremental_buffer_base)
 ut_free(incremental_buffer_base);
-if (src_file != XB_FILE_UNDEFINED)
+if (src_file != XB_FILE_UNDEFINED) {
 os_file_close(src_file);
+/* Remove .delta file after it was successfully applied.*/
+os_file_delete(0,src_path);
+}
 if (dst_file != XB_FILE_UNDEFINED)
 os_file_close(dst_file);
 return TRUE;