Bug 17246: Do no support arrayref to define multiple FK

Bug 17246: Do no support arrayref to define multiple FK

Currently you can call GetPreparedLetter like:

$prepared_letter = GetPreparedLetter(
    (
        module      => 'test',
        letter_code => 'TEST_HOLD',
        tables      => {
            reserves => [ $fk1, $fk2 ],
        },
    )
);

It assumes that $fk1 is a borrowernumber and $fk2 a biblionumber.
It seems hazardous to do this guess.

I suggest to remove this feature and only allow hashref indeed.

Test plan:
Use different way to generate letters and make sure you do not reach the croak

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>