Remove SPM2-like inference specification in snpm_pp

[cmaumet]

Currently, snpm_pp has two ways to interactively define an inference:

• Using the matlabbatch interface
• Using "SPM2-like" custom menus (c.f. https://github.com/nicholst/SnPM-devel/blob/master/snpm_pp.m#L596).

This means that we have a lot of code duplication in this function. I think that we should consider either refactoring this function to avoid code duplication or removing the "SPM2-like" option altogether. I would be in favour of the later.

@nicholst: what are your thoughts on this?