[FIR] get rid of FirFileAnnotationsContainer

This element has been introduced to simplify resolution logic in LL FIR,
but now this element is redundant and only complicates the code as after
KT-56683 `FirFile` has real phases

^KT-65876 Fixed

analysis/low-level-api-fir/testData/getOrBuildFir/annotations/fileAnnotation.txt

@@ -8,4 +8,3 @@ FIR element rendered:
 FIR FILE:
 FILE: [ResolvedTo(ANNOTATION_ARGUMENTS)] fileAnnotation.kt
     @FILE:R|kotlin/Suppress|[Types](names = vararg(String()))
-    [ResolvedTo(ANNOTATION_ARGUMENTS)] annotations container