Simplify `(T)` expressions to  `T` where possible.

Found using https://go-critic.github.io/overview.html#typeUnparen-ref

Change-Id: Ic5ef335e03898f9fea1ff90fd83956376657fe67
Reviewed-on: https://go-review.googlesource.com/123379
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>