refactor: simplify component styles by removing use-tokens

[andrewseguin]

Removes token-utils.use-tokens mixin, which was used to set global variables for the token prefixes, slots, and fallbacks that were referenced in calls to token-utils.slot.

For each component, this change:

• directly includes prefixes in token names, e.g. slide-toggle-track-width instead of just track-width
• defaults get-tokens to use a theme with system fallbacks

Before:

$token-prefix: m2-slide-toggle.$prefix;
$token-slots: m2-slide-toggle.get-token-slots();
$fallbacks: m3-slide-toggle.get-tokens(m3-system.$theme-with-system-vars);

.mdc-switch {
  @include token-utils.use-tokens($token-prefix, $token-slots, $fallbacks) {
    width: token-utils.slot(track-width);
  }
}

After:

$fallbacks: m3-slide-toggle.get-tokens();

.mdc-switch {
  width: token-utils.slot(slide-toggle-track-width, $fallbacks);
}

[andrewseguin]

The changes were merged into the following branches: main, 20.0.x

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