sqlx-cli: drop the literal style override that was invisible on light terminals (#4263)

* sqlx-cli: use cyan instead of white for help text literals

White is invisible on light terminal backgrounds. Cyan is readable on both
light and dark backgrounds, so it's a better default for the literal style.

Fixes #4112

* sqlx-cli: drop the literal style override

Default terminal color is readable on both light and dark backgrounds,
which is the original bug. No need to pick a specific replacement color.

Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>

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Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>