strconv: remove redundant type conversion

strconv: remove redundant type conversion

Change-Id: I25c8e8b701d6489f360fea30d09090826276b950
GitHub-Last-Rev: c2c8319dd5fa2a53fe82b10ecbf45eb54b5d6a68
GitHub-Pull-Request: golang/go#54924
Reviewed-on: https://go-review.googlesource.com/c/go/+/428976
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>