[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

js/js.translator/testData/box/number/kt26706.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_K2: JS_IR
 // KJS_WITH_FULL_RUNTIME
 // EXPECTED_REACHABLE_NODES: 1378
 package foo