gh-145633: Remove support for ancient ARM platforms with mixed-endian doubles (#145634)

* Drop DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754 macro.
* Use DOUBLE_IS_BIG/LITTLE_ENDIAN_IEEE754 to detect endianness of
  float/doubles.
* Drop "unknown_format" code path in PyFloat_Pack/Unpack*().

Co-authored-by: Victor Stinner <vstinner@python.org>