[thread.req.timing] Remove "note" markers around note.

This change was present in P1622R3 but the markup for the change was
missing. However, the change was requested by LWG and intended to be
made by this paper.

Author: zygoloid

source/threads.tex

@@ -109,10 +109,8 @@
 \item
 if $C_a > C_t$, the waiting function should wake as soon as possible, i.e., $C_a + D_i + D_m$,
 since the timeout is already satisfied.
-\begin{note}
 This specification may result in the total
 duration of the wait decreasing when measured against a steady clock.
-\end{note}
 
 \item
 if $C_a \leq C_t$, the waiting function should not time out until \tcode{Clock::now()} returns a