fix: correct interval calculation in tick_interval method

[crStiv]

Fix interval calculation to use INTERVALS_PER_SLOT instead of the incorrect
double modulo operation. The store.time field stores intervals since genesis,
so calculating the interval within a slot should use modulo INTERVALS_PER_SLOT
directly. This matches the formula used in tests and ensures correctness
even if SECONDS_PER_SLOT and INTERVALS_PER_SLOT values diverge in the future.

[unnawut]

I'm not sure if they're equivalent. Removing % SECONDS_PER_SLOT we need to assume that SECONDS_PER_SLOT is a multiple of INTERVALS_PER_SLOT or identical. They are coincidentally both 4 now, so modulo twice will always give the same result.

For example if SECONDS_PER_SLOT is 3 and INTERVALS_PER_SLOT remains at 4, then the current interval is different between % 4 % 4 vs % 3 % 4