CLEANUP: raw_sock: remove support for very old linux splice bug workaround

CLEANUP: raw_sock: remove support for very old linux splice bug workaround

We've been dealing with a workaround for a bug in splice that used to
affect version 2.6.25 to 2.6.27.12 and which was fixed 10 years ago
in kernel versions which are not supported anymore. Given that people
who would use a kernel in such a range would face much more serious
stability and security issues, it's about time to get rid of this
workaround and of the ASSUME_SPLICE_WORKS build option used to disable
it.