lib/deltas: Move variant read helper into the deltas code

lib/deltas: Move variant read helper into the deltas code

What the deltas code is doing is weird/unfortunate.  The name
`ot_variant_read()` conflicts too much with `ot_variant_read_fd()`.
Since nothing else uses it, move it into the deltas code.

Closes: #1254
Approved by: jlebon