x86/head/64: Remove unused GET_CR2_INTO() macro

Commit

  4b47cdbd ("x86/head/64: Move early exception dispatch to C code")

removed the usage of GET_CR2_INTO().

Drop the definition as well, and related definitions in paravirt.h and
asm-offsets.h

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201005151208.2212886-3-nivedita@alum.mit.edu