x86/tsc: Get rid of rdtscll()

Commit 99770737

 ("x86/asm/tsc: Add rdtscll() merge helper") added
rdtscll() in August 2015 along with the comment:

 /* Deprecated, keep it for a cycle for easier merging: */

12 cycles later it's really overdue for removal.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>