Simpler "use the correct symlink" checks

When determining whether to remove a symlink during unlinking, we check
three things:

  (a) Is the destination a symlink?
  (b) Does the destination exist?
  (c) Does the destination resolve to the source path?

However, since we know that the source path exists, (b) is guaranteed if
(a) and (c) are true. Thus checking (b) is unnecessary.

Similarly, when creating a new symlink during linking, we first check to
see if the link already exists by checking the same three criteria.
Again, checking (b) is unnecessary here.

See also the expanded test coverage in b52b579b.

Addendum: although we know that the source path exists during unlinking,
it doesn't matter. If the source path does not exist, then we still know
we have a broken symlink pointing into the keg we are unlinking, and
removing that symlink is still safe.