x86/boot: Remove unused finalize_identity_maps()

Commit

  8570978e ("x86/boot/compressed/64: Don't pre-map memory in KASLR code")

removed all the references to finalize_identity_maps(), but neglected to
delete the actual function. Remove it.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201005151208.2212886-2-nivedita@alum.mit.edu