Revert "boot: Add MCUBOOT_HW_KEY support for image encryption"

This reverts commit 0fa46270.

This breaks:

    samples/synchronization/sample.kernel.synchronization on
    b_u585i_iot02a/stm32u585xx/ns error

as this TF-M configuration uses its own keys.  This change is an API
change that needs to be coordinated with TF-M changes.

Before this revert, compiling this test results in:

    .../encrypted.c:447: undefined reference to
    `boot_enc_retrieve_private_key`

Signed-off-by: David Brown <david.brown@linaro.org>