x86/ftrace: Remove mcount() declaration

Commit 562e14f7 ("ftrace/x86: Remove mcount support") removed the
support for mcount, but forgot to remove the mcount() declaration.

Clean it up.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r20190826170150.10f101ba@xhacker.debian