ftrace/x86: Remove mcount() declaration

Commit 562e14f7

 ("ftrace/x86: Remove mcount support") removed the
support for using mcount, so we could remove the mcount() declaration
to clean up.

Link: http://lkml.kernel.org/r/20190826170150.10f101ba@xhacker.debian

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>