remove 15 interval RoundCash

[njason]

Addresses: #89

Original PR: #66

Not sure why 15 interval was included in the original PR as there is no information online about it. @SchumacherFM could you explain?

[SchumacherFM]

@njason This is mentioned in the description of the pull request. To recite it:

10:  10 cent rounding 3.45 => 3.50 (5 gets rounded up)
15:  10 cent rounding 3.45 => 3.40 (5 gets rounded down)

Why do you want to remove it?

[mwoss]

Firstly, RoundCash code returns incorrect values for interval = 15. We could dig in and fix it, but there is no point in doing that. I don't see any use case for such an interval. I've looked through dozens of wiki pages and other websites and I couldn't find any info about it.
Secondly, there was no real explanation of including 15 rounding interval at all.
15: 10 cent rounding 3.45 => 3.40 (5 gets rounded down) is not an explanation, imo.
From your example it looks like interval.= 15 should be used when we want round decimal to 10s, but the rounding result needs to be rounded down.
Rounding down/up logic could be useful, but it should be implemented as an additional parameter to the method signature, not as interval.

[SchumacherFM]

Thank you for the explanation. Approved ;-)

The PR should not only remove the code but also the comments indicating the 15 interval.

[mwoss]

Remove comments about interval = 15 from TestDecimal_RoundCash_Panic test too