Kernel: Remove "non-operational" ACPI parser state

If we don't support ACPI, just don't instantiate an ACPI parser.
This is way less confusing than having a special parser class whose
only purpose is to do nothing.

We now search for the RSDP in ACPI::initialize() instead of letting
the parser constructor do it. This allows us to defer the decision
to create a parser until we're sure we can make a useful one.

Kernel/ACPI/Initialize.h

@@ -29,6 +29,7 @@
 namespace Kernel {
 namespace ACPI {
 
+bool is_enabled();
 void initialize();
 
 }