vim-airline does not use actually use the `g:coc_stl_format` variable
and it also doesn't seem to be used by coc itself. So let's just remove
this code.
closes#2437
coc.nvim exposes the name of the function the cursor is currently in
through b:coc_current_function.
For this to work, the key coc.preferences.currentFunctionSymbolAutoUpdate
must be set to 'true' in the coc config.
The value is only updated on CursorHold, so the value may not always be
accurate when moving around.
