Update the locate db more reliably

[paulapatience]

Previously it would be updated only if a file (or directory) named
~/.Private existed.

• counsel.el (counsel--locate-updatedb): Replace spurious reference to
  ~/.Private with the file-name-directory of counsel-locate-db-path.

[basil-conto]

Thanks for looking into this!

Replace spurious reference to ~/.Private with the file-name-directory of counsel-locate-db-path.

However, commit b2693d1 that introduced this feature makes it look like the hard-coded ~/.Private was intentional, presumably something specific to @abo-abo's local setup. So I wouldn't be comfortable changing it without comment from @abo-abo. Furthermore, it doesn't look like counsel-locate-db-path and ~/.Private are meant to be the same thing, but then I'm not familiar with this part of Counsel.