pack/debug: remove packing identifier byte from Packdata
pack/debug: remove packing identifier byte from Packdata

Now that DirEntry has its own Packing field, this element of the
Packdata is redundant. Also, this change means users are no longer
required to stuff the byte into the Packdata for packer.Pack to work.

PackLen was doing this before this change, so the exsiting tests
now test the behavior of calling packer.Pack on a DirEntry with a
nil Packdata.

Change-Id: I50663ead01fedd998f33637293719db3ad348f85
Reviewed-on: https://upspin-review.googlesource.com/2742
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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