Revert "posix-cpu-timers: Make timespec to nsec conversion safe"

Offering: HULK
hulk inclusion
category: bugfix
bugzilla: 186339

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This reverts commit 098b0e01.

Function timespec64_to_ns() Add the upper and lower limits check in
commit c0b52741 ("time: Prevent undefined behaviour in timespec64_to_ns()"),
timespec64_to_ktime() only check the upper limits, so revert this
patch can fix overflow.

Signed-off-by: Yuyao Lin <linyuyao1@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>