Bug 35977: (follow-up) Cleaner working approach

Bug 35977: (follow-up) Cleaner working approach

This patch removes the Date_from_syspref recently added and replaces it
with the correct parseDate call as per bug 35559. We also clean up the
code around setting the input value in the first place and use iso which
is what the rest of flatpickr expects and now is handled correctly in
futuredate pickers.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 1ff3d65c7073e5d93502d329b917d886a3665e0b)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>