ctf: remove field list from constructor of StructDefinition

ctf: remove field list from constructor of StructDefinition

This makes no sense to have as structs have the field list internally.
Moreover this leads to more complicated and error prone struct initialization.
It also hides potential bugs in the tests. As highlighted in CTFEventDefinitionTest

Change-Id: I25bf18bf9be23fb2af17f52de9cf677bea1b4833
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/47992
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Hudson CI