Remove bog-citekey-function

Before, citekeys could be listed as either heading titles or properties,
but these two styles could not be mixed. Now, they can, and the
following method is used to find the citekey.

1. Take the citekey from title of the current heading.
2. Take the citekey from the property of the current heading.
3. If no citekey found, go to parent heading and repeat.

The process stops as soon as a citekey is found or a top-level heading
is reached.
Rename bog-citekey-only-p to bog-citekey-p

Old `bog-citekey-p' was not being used.
Simplify bog-citekey-p
README rephrasing
Note limitation of goto functions