cubicweb-forge #3735909 Remove multisources tests [validation pending]

Since they currently fails, and multisources is deprectaed and soon to be removed.

prioritynormal
typebug
done in1.10.2
load0.250
load left0.000
closed by#0f7eebc40e74 [test] remove test_multisources.py (closes #3735909)