remove qemu_get_clock

These patches are already not doing a great service to out-of-tree
modifications to QEMU.  However, at least we can warn them by getting
rid of the old confusing functions, or otherwise causing compilation
errors.  This patch removes qemu_get_clock; the previous one changed
qemu_new_timer's signature.

Signed-off-by: Paolo Bonzini <[email protected]>