The pprof tools properly cleans up all files it creates, but forgets
to clean up the temporary directory itself. This CL fixes that.

Fixes #13863

Change-Id: I1151c36cdad5ace7cc97e7e04001cf0149ef0f63
Reviewed-on: https://go-review.googlesource.com/23019Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>