[LL] CleanableSoftValueCache: Avoid cleanup on already removed values

- The cache guarantees deterministic cleanup on removal, so if the
  reference has already been removed from the cache, there is no need to
  clean it up again.
- Still, I don't want to guarantee that `SoftValueCleaner` is only
  invoked once (see its documentation), as soft reference semantics
  cannot be tested properly in our current test infrastructure.

^KT-61222

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/caches/CleanableSoftValueCache.kt

@@ -61,8 +61,12 @@ class CleanableSoftValueCache<K : Any, V : Any>(
             @Suppress("UNCHECKED_CAST")
             ref as SoftReferenceWithCleanup<K, V>
 
-            backingMap.remove(ref.key, ref)
-            ref.performCleanup()
+            val wasRemoved = backingMap.remove(ref.key, ref)
+
+            // If `ref` already wasn't part of the map, it will have been cleaned up by a deterministic removal operation.
+            if (wasRemoved) {
+                ref.performCleanup()
+            }
         }
     }