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ARROW-10448: [Rust] Remove PrimitiveArray::new that can cause UB

This PR removes `PrimitiveArray::new`. `PrimitiveArray::new` is `pub`, but it is dangerous because:
* when the buffer's content is not aligned with T, many of its methods cause UB
* when used with `null_count > 0`, many calls panic as the null bitmap is `None`, but the `null_count != 0`
* when used with `null_count > 0`, it creates an array out of spec (as the buffer for the null bitmap is `None` but the null count is not zero)

Since:
* a change in this method's signature (to either add the bitmap or remove `null_count`) requires a backward incompatible change
* it is only used in tests
* we have good offers to create primitive arrays:
    * from an ArrayData,
    * from a vector or vector of optionals
    * from an iterator

This PR removes it.

Closes #8560 from jorgecarleitao/remove_new

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>