misc/makerelease: use built in "del" to remove files

Git marks some of its files read only, so os.RemoveAll isn't sufficient
to remove them from the ".git" directory.

Change-Id: I3150596931d1c77e7cf9fb8da1a999d2c6730121
Reviewed-on: https://go-review.googlesource.com/2930


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>