refactor: TopLevel implicit is now allowed

It seems like TopLevelCantBeImplicit is no longer the case as of #5754
And it is actually confirmed in https://github.com/lampepfl/dotty/blob/93fc41fcb624df73cc12d52b79d518a30a778a7c/tests/run/toplevel-implicits/a.b.scala#L19-L21

This commit replace the unnecessary check in from Checking.scala to
assertion and deleted the `ErrorMessage` definition for `TopLevelCantBeImplicit`.

I'm leaving the `TopLevelCantBeImplicitID` in `ErrorMessageID.scala` so
we don't screw up the error number.

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1510,13 +1510,6 @@ import transform.SymUtils._
     def explain = ""
   }
 
-  class TopLevelCantBeImplicit(sym: Symbol)(
-    implicit ctx: Context)
-    extends SyntaxMsg(TopLevelCantBeImplicitID) {
-    def msg = em"""${hl("implicit")} modifier cannot be used for top-level definitions"""
-    def explain = ""
-  }
-
   class TypesAndTraitsCantBeImplicit()(using Context)
     extends SyntaxMsg(TypesAndTraitsCantBeImplicitID) {
     def msg = em"""${hl("implicit")} modifier cannot be used for types or traits"""

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -473,8 +473,7 @@ object Checking {
     if sym.isInlineMethod && !sym.is(Deferred) && sym.allOverriddenSymbols.nonEmpty then
       checkInlineOverrideParameters(sym)
     if (sym.is(Implicit)) {
-      if (sym.owner.is(Package))
-        fail(TopLevelCantBeImplicit(sym))
+      assert(!sym.owner.is(Package), s"top-level implicit $sym should be wrapped by a package after typer")
       if sym.isType && (!sym.isClass || sym.is(Trait)) then
         fail(TypesAndTraitsCantBeImplicit())
     }