Remove C-n and C-p from the insert mode keymap (#3340)

These are read-line-like bindings which we'd like to minimize in
insert mode in general.

In particular these two are troublesome if you have a low
`editor.idle-timeout` config and are using LSP completions: the
behavior of C-n/C-p switches from moving down/up lines to moving
down/up the completion menu, so if you hit C-n too quickly
expecting to be in the completion menu, you'll end up moving down
a line instead. Using C-p moves you back up the line but doesn't
re-trigger the completion menu. This kind of timing related change
to behavior isn't realistically that big of a deal but it can be
annoying.

helix-term/src/keymap/default.rs

@@ -358,9 +358,7 @@ pub fn default() -> HashMap<Mode, Keymap> {
         "left" => move_char_left,
         "C-b" => move_char_left,
         "down" => move_line_down,
-        "C-n" => move_line_down,
         "up" => move_line_up,
-        "C-p" => move_line_up,
         "right" => move_char_right,
         "C-f" => move_char_right,
         "A-b" => move_prev_word_end,