Revert deprecation of PERL6_EXCEPTIONS_HANDLER

The PERL6_EXCEPTIONS_HANDLER environment variable is actually part of the 6.d
spec, so we cannot remove it. Further the deprecation message went into the same
stream as the supposedly machine readable output, potentially disrupting tools
that this output was meant for.

src/core.c/Exception.pm6

@@ -377,10 +377,6 @@ do {
         my $e := EXCEPTION($ex);
 
         if %*ENV<PERL6_EXCEPTIONS_HANDLER> -> $handler {
-            # REMOVE DEPRECATED CODE ON 202011
-            Rakudo::Deprecations.DEPRECATED: "RAKU_EXCEPTIONS_HANDLER", Nil,
-                '2020.11', :file("N/A"), :line("N/A"),
-                :what("PERL6_EXCEPTIONS_HANDLER env var");
             my $class := ::("Exceptions::$handler");
             unless nqp::istype($class,Failure) {
                 temp %*ENV<PERL6_EXCEPTIONS_HANDLER> = ""; # prevent looping