terminal: do not reset airline theme on TerminalOpen
terminal: do not reset airline theme on TerminalOpen

remove autocommand, that would overwrite the airline theme on :term

(e.g. After a :AirlineTheme monochrome and then using `:term` the
autocommand would reset the theme back to the one given in the .vimrc)

rchicoli/vim-airline,yzhou61/vim-airline,bling/vim-airline,chrisbra/vim-airline