Fixes: https://github.com/zed-industries/community/issues/1712

The keymaps were adding in a `"cmd-enter": "editor::NewlineBelow",`
entry in the context of `Editor`, and this was clobbering the assist
command in the assistant panel context. Zed now defines this command in
the default keymap under the context of `"context": "Editor && mode ==
full"`. All I needed to basically do was remove that command from the
keymaps. I also removed the `"cmd-shift-enter": "editor::NewlineAbove"
from the `Editor` context in those keymaps as wel, as it is also defined
in the default keymap.

Release Notes:

- Fix bug preventing the `assistant: assist` command from working in
certain keymaps