x86/asm: Drop unused RDPID macro

Drop the GAS-compatible RDPID macro. RDPID is unsafe in the kernel
because KVM loads guest's TSC_AUX on VM-entry and may not restore the
host's value until the CPU returns to userspace.

See

  6a3ea3e6 ("x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM")

for details.

It can always be resurrected from git history, if needed.

 [ bp: Massage commit message. ]

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20201027214532.1792-1-sean.j.christopherson@intel.com