core/ui.el: Disable the custom default mode-line

Setting a default mode-line will confuse smart-mode-line and other
packages, don't do that, we don't need any of the customisation there
anyway.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
users/algernon[@hadhodrond]: Enable nyan-mode too

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
users/algernon[@hadhodrond]: Disable diminished mode

Lets experiment with undiminished mode and smart-mode-line for a while.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>