dir/dircache: remove hack to prevent caching anyone else's directory
dir/dircache: remove hack to prevent caching anyone else's directory

The hack went in before we were using the Watch interface because
the polling would take too long to notice changes. We no longer
need it.

Change-Id: I3210c51ab3a7884337ea3a3a5f1e9ec7e7c58d13
Reviewed-on: https://upspin-review.googlesource.com/8760
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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