x86/apic: Nuke empty init_apic_ldr() callbacks

apic::init_apic_ldr() is only invoked when the APIC is initialized. So
there is really no point in having:

  - Default empty callbacks all over the place

  - Two implementations of the actual LDR init function where one is
    just unreadable gunk but does exactly the same as the other.

Make the apic::init_apic_ldr() invocation conditional, remove the empty
callbacks and consolidate the two implementation into one.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)