vim-intellij-client: Use omnifunc instead of completefunc.

Omnifunc is better. It gives you, for example, integration with
YouCompleteMe right out of the box.

By now, just for some sample file types (java, javascript).
vim-intellij-client: Set resolve mapping only for supported filetypes.

By now, just for some sample file types (java, javascript).
vim-intellij-client: Remove unused settings.

These aren't doing anything by now.
Better remove them and reintroduce them when, and if implemented.
vim-intellij-client: Autoactivate when within IDEA project.

Instead of having autocompletion and resolving activated only for some
specific filetypes (hardcoded by now), activate them automatically for
all files, but only when within an idea project.