accounts/abi: simplify reflection logic (#21058)

* accounts/abi: simplified reflection logic

* accounts/abi: simplified reflection logic

* accounts/abi: removed unpack

* accounts/abi: removed comments

* accounts/abi: removed uneccessary complications

* accounts/abi: minor changes in error messages

* accounts/abi: removed unnused code

* accounts/abi: fixed indexed argument unpacking

* accounts/abi: removed superfluous test cases

This commit removes two test cases. The first one is trivially invalid as we have the same
test cases as passing in packing_test.go L375. The second one passes now,
because we don't need the mapArgNamesToStructFields in unpack_atomic anymore.
Checking for purely underscored arg names generally should not be something we do
as the abi/contract is generally out of the control of the user.

* accounts/abi: removed comments, debug println

* accounts/abi: added commented out code

* accounts/abi: addressed comments

* accounts/abi: remove unnecessary dst.CanSet check

* accounts/abi: added dst.CanSet checks