runtime: drop padding alignment field for timeHistogram

runtime: drop padding alignment field for timeHistogram

After CL 419449, timeHistogram always have 8-byte alignment.

Change-Id: I93145502bcafa1712b811b1a6d62da5d54d0db42
Reviewed-on: https://go-review.googlesource.com/c/go/+/425777
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>