Remove `<C-x><C-t>` mapping and encourage people to use 'completefunc' instead

[hauleth]

This would simplify setup and minisnip#complete function

[arp242]

The problem with this is that people may already use completefunc for the filetype-specific completion. I don't think we want to override that for something comparatively minor as snippet completion. That's the reason I implemented it as it is now.

[hauleth]

@Carpetsmoker even then it would be much better IMHO to provide support for using minisnip as 'completefunc' for those who use 'omnifunc' for filetype completion. It could be done quite simply, so I will try to write some PR soon.