CompatibilityMode cleanup

Review: https://jetbrains.team/p/kt/reviews/8401

- Drop unused variables
- Replace `assert` with `require` because `assert` isn't reliable which
  makes hard to do any judgements about the code (`-ea` flag must be
  passed to the VM for the `assert` to be active. And it's hard to know
  when we the flag is passed). If `assert` was here for the performance
  reasons then a comment should have been left.
- Add the assert message for clearity

Because I'm evaluating what will break if KotlinAbiVersion is bumped.
And when the code around KotlinAbiVersion is clean it's easier to do the
evaluation.

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/IrModuleDeserializer.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.ir.symbols.*
 import org.jetbrains.kotlin.ir.util.IdSignature
 import org.jetbrains.kotlin.library.IrLibrary
 import org.jetbrains.kotlin.library.KotlinAbiVersion
+import org.jetbrains.kotlin.library.KotlinLibraryProperResolverWithAttributes
 
 fun IrSymbol.kind(): BinarySymbolData.SymbolKind {
     return when (this) {
@@ -31,7 +32,9 @@ fun IrSymbol.kind(): BinarySymbolData.SymbolKind {
 class CompatibilityMode(val abiVersion: KotlinAbiVersion) {
 
     init {
-        assert(abiVersion.isCompatible())
+        require(abiVersion.isCompatible()) {
+            "Incompatible KLIB should have been discarded in ${KotlinLibraryProperResolverWithAttributes<Nothing>::libraryMatch.name}"
+        }
     }
 
     val oldSignatures: Boolean
@@ -44,11 +47,7 @@ class CompatibilityMode(val abiVersion: KotlinAbiVersion) {
 
     companion object {
         val LAST_PRIVATE_SIG_ABI_VERSION = KotlinAbiVersion(1, 5, 0)
-
-        val WITH_PRIVATE_SIG = CompatibilityMode(LAST_PRIVATE_SIG_ABI_VERSION)
-        val WITH_COMMON_SIG = CompatibilityMode(KotlinAbiVersion.CURRENT)
-
-        val CURRENT = WITH_COMMON_SIG
+        val CURRENT = CompatibilityMode(KotlinAbiVersion.CURRENT)
     }
 }