internal/poll: eliminate the redundant conditional branch for isKernelVersionGE53

internal/poll: eliminate the redundant conditional branch for isKernelVersionGE53

Follow up CL 573755

Change-Id: I27c7571d3ef1274cf2c6892e678f946f9b65de33
Reviewed-on: https://go-review.googlesource.com/c/go/+/576416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>