date: strip the O strftime modifier in the C locale

[igorgbr]

Fixes #11656

In the C locale the O strftime modifier requests alternative numeric
symbols that do not exist, so GNU date treats it as a no-op: %Om
renders exactly like %m. jiff does not know the modifier and emitted it
literally.

Strip the modifier from the format string before jiff sees it, mirroring
the existing substitute_epoch_seconds pass. %% is preserved so that an
O following a literal percent stays plain text.

Before:

    $ LC_ALL=C TZ=UTC date -d 2024-06-15 +%Om-%Oy-%Ol
    %Om-%Oy-%Ol

After (matches GNU):

    $ LC_ALL=C TZ=UTC date -d 2024-06-15 +%Om-%Oy-%Ol
    06-24-12

The test for this behaviour already existed in tests/by-util/test_date.rs
marked #[ignore]; this PR removes the ignore.