Remove unused rz_big

[wargio]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Its is unused and slims down the libs

[XVilka]

Wait, it might be useful for the big numbers in rz-ax, as well as in RzIL

[XVilka]

See, e.g. #3331

[wargio]

Wait, it might be useful for the big numbers in rz-ax, as well as in RzIL

i don't see the need for this. BitVec is enough also for rz-ax.
Besides, is not used anywhere, besides on the unit test

[XVilka]

Performing big number arithmetic by converting to bitvectors back and forth where there is no need for them will be very slow. Please pause this PR, we need better discussion about it