tabline: remove static caching of s:vars
tabline: remove static caching of s:vars

rather let them be defined whenever they are needed. They were only used
inside a single function anyhow, so it does not make sense to cache
them. In addition, having the user later change the variable won't work
as expected.

tschundler/dotfiles,bling/vim-airline,rchicoli/vim-airline,yzhou61/vim-airline,tschundler/dotfiles,tschundler/dotfiles,unphased/vim-airline,chrisbra/vim-airline