content: Remove TextBaseline.alphabetic from plain-paragraph style

I'm not sure if this actually improves how we show non-alphabetic
text in any cases at all (so I'm not sure if it's NFC), but anyway,
hard-coding this attribute here or anywhere has a smell of being
counterproductive for i18n; remove it.

lib/widgets/content.dart

@@ -39,7 +39,6 @@ class ContentTheme extends ThemeExtension<ContentTheme> {
       color: const HSLColor.fromAHSL(1, 0, 0, 0.15).toColor(),
       fontSize: kBaseFontSize,
       letterSpacing: 0,
-      textBaseline: TextBaseline.alphabetic,
       height: (22 / kBaseFontSize),
       leadingDistribution: TextLeadingDistribution.even,
       decoration: TextDecoration.none,