fix: don't screen unit for a Unitful unit

    `__get_literal_unit` already screens units of DynamicQuantity.AbstractQuantity. So this check is redundant.
    Also, removing this check fixes the issue with `get_unit` for units of Unitful type.

Author: ven-k

src/systems/unit_check.jl

@@ -126,7 +126,7 @@ end
 
 function get_unit(x::Symbolic)
     if (u = __get_literal_unit(x)) !== nothing
-        screen_unit(u)
+        u
     elseif issym(x)
         get_literal_unit(x)
     elseif isadd(x)