KVM: x86: drop fpu_activate hook

KVM: x86: drop fpu_activate hook

The only user of the fpu_activate hook was dropped in commit
2d04a05bd7e9 (KVM: x86 emulator: emulate CLTS internally, 2011-04-20).
vmx_fpu_activate and svm_fpu_activate are still called on #NM (and for
Intel CLTS), but never from common code; hence, there's no need for
a hook.

Reviewed-by: Yang Zhang <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>