gettimeofday.2: Do mention improbable future removal

As kib@ noted:

Obviously gettimeofday(2) is not going to be removed
even in the far future.

Reported by: kib
Fixes: 4395d3ced5cf Document that gettimeofday() is obsolescent
MFC after: 3 days

(cherry picked from commit 6662c2312e956439652ce2d06b42753b6a78fc61)