Remove mishandled erc-control-default-{fg,bg} faces

Remove mishandled erc-control-default-{fg,bg} faces

Partially revert those portions of 7b4ca9e609e "Leverage inverse-video
for erc-inverse-face" that introduced and managed explicit faces for
the "default" 99 color code.

* lisp/erc/erc-goodies.el (erc-control-default-fg)
(erc-control-default-bg): Remove unused faces originally meant to be
new in ERC 5.6.
(erc-get-fg-color-face, erc-get-bg-color-face): Return nil for n=99.
(erc-controls-interpret, erc-controls-highlight): Preserve an
interval's existing background so "if only the foreground color is
set, the background color stays the same," as explained by
https://modern.ircdocs.horse/formatting#color.  (Bug#69860)

Copyright-paperwork-exempt: yes
(cherry picked from commit 525bc083155030b58de08c8716fec9db1496aa9d)