K2: minor, remove misleading FirMemberDeclaration.containerSource

"Container source" means "the source of the container". So, if you
called this extension on a class (`FirClassLikeDeclaration`), it was
reasonable to assume that you would get the source of its _containing
class_, whereas the function actually returned the source of the class
itself.

Instead of inventing another name for this function, I've decided to
inline and remove it because it has only one usage.

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/utils/declarationAttributes.kt

@@ -49,12 +49,6 @@ var <T> T.danglingTypeConstraints: List<DanglingTypeConstraint>?
 
 // ----------------------------------- Utils -----------------------------------
 
-val FirMemberDeclaration.containerSource: SourceElement?
-    get() = when (this) {
-        is FirCallableDeclaration -> containerSource
-        is FirClassLikeDeclaration -> sourceElement
-    }
-
 val FirProperty.hasExplicitBackingField: Boolean
     get() = backingField != null && backingField !is FirDefaultPropertyBackingField