Revert "Simple hack/fix to allow tab in insert mode when completing function and template arguments"

This reverts commit d9dccefca96f95a1d1543425dec33fbfadd7daf0.

This caused conflicts with supertab and ultisnips. I revert it for now.

I still like this functionality, but we obviously need to design it
better.
Merge pull request #409 from tobig/revert-tab-in-insert-mode

Revert "Simple hack/fix to allow tab in insert mode when completing func...