timeline: only use the send queue mechanism to send reactions

[bnjbvr]

Independently of #3749, we can, as of today, start using the send queue only to send reactions, instead of having the logic of sending reactions in the timeline itself

There was a mechanism to queue adding/removing a reaction, with the possibility to get observable updates for each add/remove that occurred. In some contexts, if you quickly tapped a reaction button on ElementX apps, you could observe it: the reaction would be added, then removed, then added back, etc. until it stabilized.

I don't see the user value for this: if I quickly tap the reaction button, I'd expect the sending of the reaction to be aborted, if it can, instead of delayed into a redaction event that's subsequently sent. The sending queue allows doing this for free, so making use of that.

I've also removed tests which value was also unclear or suspect.

[codecov]

Codecov Report

Attention: Patch coverage is 82.70677% with 23 lines in your changes missing coverage. Please review.

Project coverage is 84.10%. Comparing base (c04dd18) to head (0723d4c).
Report is 5 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/inner/mod.rs	74.71%	22 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/util.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3865      +/-   ##
==========================================
- Coverage   84.11%   84.10%   -0.02%     
==========================================
  Files         266      266              
  Lines       27787    27724      -63     
==========================================
- Hits        23372    23316      -56     
+ Misses       4415     4408       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Hywan]

I like the way it goes, and it's clearly an improvement. Less code, more re-used code, well done!

I wonder if the patch could be split into multiple smaller ones though, it's not easy to review, but generally the changes look good! I'm confident with the tests.