Fix or remove the "add step" context option on Beat clips

### Enhancement Summary

Since #8369 all tracks in a pattern have a clip of the same shared length.

Yet, there are still some ways remaining to change the length of a clip without changing the length of its pattern : Copy+Paste (that should be taken care of by #8512) and the "add steps" option of Beat type clips context menu.

### Implementation Details / Mockup

I see two ways to resolve this issue:

_**1) Fix the option so the length of the pattern is updated**_
This involve adding code which purpose will not be easily understandable and could be the source of potential regressions in future updates.

**_2) Just remove the option_**
This is the simpler way (and imo the better). The context menu is huge and could benefit from some pruning, and there is already dedicated buttons in the pattern editor's ui.

What do you think about this ?

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by locating the Beat clip context-menu implementation and the "add steps" action, then compare it with the dedicated pattern editor controls mentioned in the issue. Done means the option is removed or updates the pattern length consistently, without leaving a way to change a clip length independently.

Written by the indexing model from the issue text.