kernel: Remove unused xwait headers.

kernel: Remove unused xwait headers.

xsleep/xwakeup were added to DragonFly in 2003 to replace asleep/awakeup
from FreeBSD 4.x. tsleep & wakeup sleep on untyped identifiers; the sleep
routines do not store through the identifiers and treat them as opaque.
xsleep/xwakeup in contrast slept on explicit sleep structures,
'struct xwait'.

xsleep / xwakeup were never used; they were born #if-0ed out and were
removed by 2004. This commit removes a legacy declaration and associated
header.

Submitted-by:  vsrinivas
Dragonfly-bug: <https://bugs.dragonflybsd.org/issues/2786>