remove unused and confusing float16 constants (#21999)
### Description
Remove unused and confusing special constants in MLFloat16 and BFloat16
types.
### Motivation and Context
While looking at adding a specialization for std::numeric_limits for the
16-bit floating point types, I found that there are various special
constants in those types that are confusing or just wrong.
MLFLoat16::Epsilon is not an epsilon at all, but approximates "e". Looks
like a copy-paste bug.
BFloat16::Epsilon does not correspond to `numeric_limits::epsilon()`,
nor even to the C# Float.Epsilon.
Instead, it corresponds to `numeric_limits::min()` which was really
confusing to me.
The "MinValue" constants does correspond to the C# `Float.MinValue`
constant, but this is C++ so it would be better renamed to "LowestValue"
since it corresponds to `numeric_limits::lowest()`. As it was unused
except for some unit tests I have replaced it with the equivalent
`MaxValue.Negate()` here.
There's also an unused `kSignaling_NaNBits` constant which is just wrong
(has the same value as `kPositiveInfinityBits` instead of a NaN).
