debug/buildinfo: remove redundant type conversion

debug/buildinfo: remove redundant type conversion

Change-Id: Ia8ffc9a17646ad2c74f633610aaf34fef006c023
GitHub-Last-Rev: 6ab0d107393c9bdfeaed5c539efb452942c00394
GitHub-Pull-Request: golang/go#54856
Reviewed-on: https://go-review.googlesource.com/c/go/+/428235
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>