[FIR JS] KT-57601: Get rid of the builtin provider

In this example we get 2 variants of `kotlin/toString()`:
one coming from the KLib provider, and the
other coming from the builtins provider.
These are identical deserialized functions.

^KT-57601 Fixed

compiler/testData/cli/js/firMppWithKlib.args

@@ -9,3 +9,5 @@ $TEMP_DIR$
 firMppWithKlib
 -XXLanguage\:+MultiPlatformProjects
 -Xir-produce-klib-file=true
+-libraries
+libraries/stdlib/js-ir/build/classes/kotlin/js/main