Remove heuristic to detect macro invocation via Ext method

[mooori]

PR959 in near-sdk-rs stops forwarding of attributes (other than cfg) to _Ext methods. Once we use a near-sdk version including that change, the heuristic to detect if near-plugin macros are invoked via _Ext methods can be removed. This refers to condition_2 here:

https://github.com/aurora-is-near/near-plugins/blob/7454bbcfcdb50addee5fefe3742879d1fac75c81/near-plugins-derive/src/utils.rs#L25-L58