refactor: remove automock to prevent must_use warnings for impl

[severin-amrein]

This PR was caused by warnings of the following structure:

warning: `#[must_use]` attribute cannot be used on trait methods in impl blocks
   --> rs/cli/src/forum/mod.rs:148:5
    |
148 |     #[must_use = "You must not forget to update the proposal URL using the forum post this returns"]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = help: `#[must_use]` can be applied to data types, functions, unions, required trait methods, provided trait methods, inherent methods, foreign functions, and traits
    = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default

They are caused by using automock that automatically creates mock impl including must_use. Since the mock functions are not used at the moment, this PR removes automock.

[NikolaMilosa]

Automock is used for testing.

Given that we don't have tests with this, and the CI is green I see no issues with merging this.