version: don't have a third version number form for xcode

Our primary version format is git describe --long --abbrev=9.

Our Apple scheme is:
    (major+100).minor.(patch*10,000+gitDescribeCommits).

This CL gets rid of the third, which was:
    major.minor.(patch*10,000+gitDescribeCommits).

Now the "About" box in the macOS app shows the same version that we
show on pkgs.tailscale.com, userz, changelog, etc.

This will be more important once/if we get standalone DMG downloads
for macOS on pkgs.tailscale.com.

Fixes tailscale/corp#364
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