Revert parameter underlining on default theme

I like it, but it clashes with diagnostics underlines since we can't
color them differently in the terminal. If undercurl support is
sufficient enough I'd consider changing diagnostics to use that instead.

theme.toml

@@ -9,7 +9,8 @@ special = "honey"
 property = "white"
 variable = "lavender"
 # variable = "almond" # TODO: metavariables only
-"variable.parameter" = { fg = "lavender", modifiers = ["underlined"] }
+# "variable.parameter" = { fg = "lavender", modifiers = ["underlined"] }
+"variable.parameter" = { fg = "lavender" }
 "variable.builtin" = "mint"
 type = "white"
 "type.builtin" = "white" # TODO: distinguish?