Remove/replace check for 'set_balloons' in ale#balloon#MessageForPos

These days many projects support balloons which is awesome, but they are all overriding the `set balloonexpr=...`
for example:
`vim-go - go#tool#DescribeBalloon()`
`ale - ale#balloon#Expr()`
also I have my own custom balloon logic for previewing folds.

It would be nice if `ale#balloon#MessageForPos` can return messages without checking
`let g:ale_set_balloons = 1` because it will override the `set balloonexpr=...` (maybe some other variable can be introduced)

This way we'll be able to write our own balloonexpr func and choose when to show contents of folds, when linter warnings and when golang definitons.