drop dependency on axios

[giogonzo]

I think there are no valid reasons to continue requiring axios for this library:

• generated API is never used on node -> no need for a lib supporting both browser and node adapters
• fetch has been standard in browsers for a while, and the functionality of this lib can be achieved just fine using it directly
• we'd drop a dependency which is noisy in terms of security alerts https://github.com/buildo/metarpheus-io-ts/network/alert/package.json/axios/open