jvm-abi-gen: Remove empty FileClasses from abi

[Tagakov]

These two code snippets have different abi:

//file test.kt
private fun funToBeRemoved() = Unit

class Class

and

//file test.kt
class Class

It happens because of two reasons:

• Empty (absolutely, even zero constructors) FileClass TestKt is not being removed from the abi.jar
• kotlin_module file has a record for mentioned FileClass as well

Meaning that adding/removing first/last top-level function in a file invalidates ABI in 100% cases.

I've tried to fix the issue, but can't make kotlin_module the same for two snippets above.

Youtrack ticket: https://youtrack.jetbrains.com/issue/KT-65696/