Remove async playground doits

[guillep]

As they can cause lots of unexpected concurrency issues.
They are a nice idea but they need more work.

See for example pharo-project/pharo#16311

More importantly, this breaks profiling completely, and has unexpected results that even experienced users will have difficulties reasoning about.

Here is an example of the potential performance issues we could have with a small example.
The async do it executes in low priority, which contends against the UI thread, giving overheads of at least ~4x in the following silly example.
And we have observed last week even worse cases and deadlocks.

So leaving a button that can deadlock your image is not very nice unless it has a dead head :)

Normal

[1000 timesRepeat: [ 
		1700 factorial.
		Transcript show: 'x'; cr ]
] timeToRun.

[AndreasSystemProfiler spyOn: [ 
	1000 timesRepeat: [ 
		1700 factorial.
		Transcript show: 'x'; cr ]
]] timeToRun.

"Async Do-it"		1.338s.
"Async+profiler"	1.818s.
"Sync Do-it"			0.569s.
"Sync+profiler"		0.706s.

High

[ t := [1000 timesRepeat: [ 
		1700 factorial.
		Transcript show: 'x'; cr ]
] timeToRun.] forkAt: 41.

[ t := [AndreasSystemProfiler spyOn: [ 
	1000 timesRepeat: [ 
		1700 factorial.
		Transcript show: 'x'; cr ]
]] timeToRun ] forkAt: 41.

"Async Do-it"		0.568s.
"Async+profiler"	0.708s.
"Sync Do-it"			0.582s.
"Sync+profiler"		0.713s.