Remove obsolete assertion in ExpressionCodegen

With the mangling added in 488418d960, there's no longer any risk in
writing "special" function name ("<anonymous>" in this case) to the
local variable table.

 #KT-34356 Fixed

compiler/testData/codegen/box/functions/functionExpression/insideGenericLambda.kt

@@ -0,0 +1,9 @@
+fun <T> block(block: () -> T): T = block()
+fun foo() {}
+
+fun test(): () -> Unit = block { fun() = foo() }
+
+fun box(): String {
+    test()
+    return "OK"
+}