transpile: Do not try to eliminate casts from integer literal to enum

[Rua]

• Depends on transpile: Translate enums with newtype structs #1620

c2rust has for a long time created a "shortcut" if an integer literal is cast to an enum type, first added in e9e7e46. Instead of translating each of them as-is, it would attempt to emit an enum constant and fall back to an integer literal otherwise. But I think that's bad:

• If the original source code contained an integer literal rather than an enum constant, it may be more faithful to preserve that.
• As noted in transpile: Translate enums with newtype structs #1620 (comment), that shortcut causes problems if the literal doesn't fit into the integral type of the enum.

So I think it's best to just remove it and emit the literal and cast as they are.