refactor(tvix/eval): merge OpCall & OpTailCall

As applies are thunked, there was no situation where OpCall could be
emitted. In practice, all calls were already tail calls.

Change-Id: Id0d441dcdd86f804d7cddd0cc14f589bbfc75e5b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8147
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/opcode.rs

@@ -141,7 +141,6 @@ pub enum OpCode {
 
     // Lambdas & closures
     OpCall,
-    OpTailCall,
     OpGetUpvalue(UpvalueIdx),
     /// A Closure which has upvalues but no self-references
     OpClosure(ConstantIdx),