fix(md): item border has improved contrast in dark mode (#29398)

Issue number: resolves #29386

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## What is the current behavior?
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The item border does not have sufficient contrast in dark mode making
the border almost invisible.

## What is the new behavior?
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- Removed the `--ion-border-color` in the MD dark palette. Instead,
we're using the [theme
default](dc1172a841/core/src/themes/ionic.theme.default.md.scss (L31)).
## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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The earliest reference to the offending line Brandy and I could find was
[here](https://github.com/ionic-team/ionic-framework/issues/18713#issuecomment-510644371).

core/src/css/palettes/dark.scss

@@ -142,7 +142,6 @@ $colors:  (
     --ion-background-color-rgb: 18, 18, 18;
     --ion-text-color: #ffffff;
     --ion-text-color-rgb: 255, 255, 255;
-    --ion-border-color: #222222;
     --ion-background-color-step-50: #1e1e1e;
     --ion-background-color-step-100: #2a2a2a;
     --ion-background-color-step-150: #363636;