.vimrc: Cleanup 'highlight' overrides

.vimrc: Cleanup 'highlight' overrides

The current colorscheme should set all the standard 'highlight'
groups. No need to override them locally.

- Remove all 'guifg'/'guibg' overrides, keep only terminal-type
  overrides.  GUI sessions (e.g. gvim) should use 24-bit colors
  as defined in current `colorscheme`.

- Remove 'StatusLine' highlights. Everything status-line is handled
  by `vim-airline`.

- Only define override 'SpecialKeys' highlights if we're using a
  16-color $TERM [because I don't like the default solarized color-
  scheme 'SpecialKeys' highlight colors].

- Remove 'Todo' highlight. The current `colorscheme` should define
  this, no need to override it.