contracts: Remove unused benchmarks (#3185)

Those were used for some adhoc comparison of solang vs ink! with regards
to ERC20 transfers. Not been used for a while.

Benchmarking is done here now:
[smart-bench](https://github.com/paritytech/smart-bench): Weight based
benchmark to test how much transaction actually fit into a block with
the current Weights
[schlau](https://github.com/ascjones/schlau): Time based benchmarks to
compare performance