Fix #61, remove dependence on ENDIAN macro for checksum

[jphickey]

Describe the contribution
Removes the checksum "optimization" for big endian, as it is unlikely to be useful and creates endianness dependencies.

Fixes #61

Testing performed
Build and sanity check CF

Expected behavior changes
None on x86/little endian (removes code that was not used)
PPC/big endian will use the same code as x86, might be a bit slower but its safer and more testable (because its the same code)

System(s) tested on
Ubuntu 21.10

Additional context
The big endian optimization is probably not that useful because it requires that the HW can do misaligned 32-bit reads. AFAIK most embedded CPUs do not have this capability, and if they can, it might not be all that much faster than reading it byte-by-byte. So even though it looks like a 32-bit read in C code (and therefore 4x fewer loop iterations) it probably doesn't have much of a payoff, if one could find a hardware with this capability (not sure how it was originally tested). In the end I don't think its worth trying to test and maintain this "optimization".

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[skliper]

Absolutely agree 100% this is a critical improvement.

[astrogeco]

CCB:2022-01-19 APPROVED

• Removes platform-specific stuff and optimization for big endian systems
• Ensures code is endian agnostic
• See checksum optimization implementation path in Support for alternate checksum type indicated in Metadata PDU #99