bitbake.conf: Drop export of SOURCE_DATE_EPOCH_FALLBACK

Whilst SDE definitely needs to be exported, the fallback does not as
it is only used in our python code via the datastore.

It was introduced as an export in 9a1dde74
but even then it doesn't look like it needed to be, likely just a copy and
paste mistake.

Drop the export.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>