flex: Regen bootstrap files

This also partially reverts r326025 (8a16b7a18f5d). I do not see any
point of adding SPDX tag in generated file.

MFC after: 3 days
Submitted by: Dan McGregor <dan.mcgregor@usask.ca> (initial version)
Differential Revision: https://reviews.freebsd.org/D28596