[FIR] Remove FirLambdaArgumentExpression

It's not really necessary if the information about if the lambda was a
trailing lambda can be directly saved in FirAnonymousFunctionExpression.

Removing the FIR node uncovered a couple of bugs
(UNINITIALIZED_ENUM_ENTRY, ERROR_IN_CONTRACT_DESCRIPTION) that were
caused by assuming that a lambda is always a trailing lambda.

#KT-66124

analysis/low-level-api-fir/testData/getOrBuildFir/expressions/lambdaExpression.txt

@@ -3,7 +3,7 @@ FIR element: FirAnonymousFunctionExpressionImpl
 FIR source kind: KtRealSourceElementKind
 
 FIR element rendered:
-[ResolvedTo(BODY_RESOLVE)] [MatchingParameterFunctionTypeKey=kotlin/Function0<R>] run@fun <anonymous>(): R|kotlin/Int| <inline=Inline, kind=EXACTLY_ONCE>  {
+<L> = [ResolvedTo(BODY_RESOLVE)] [MatchingParameterFunctionTypeKey=kotlin/Function0<R>] run@fun <anonymous>(): R|kotlin/Int| <inline=Inline, kind=EXACTLY_ONCE>  {
     ^ Int(1)
 }