x86/ftrace: Remove unused modifying_ftrace_code declaration

All uses of modifying_ftrace_code have been removed by
commit 768ae440

 ("x86/ftrace: Use text_poke()"),
so remove the declaration, too.

Link: https://lkml.kernel.org/r/20220914110437.1436353-2-cuigaosheng1@huawei.com

Cc: <mingo@redhat.com>
Cc: <tglx@linutronix.de>
Cc: <bp@alien8.de>
Cc: <dave.hansen@linux.intel.com>
Cc: <x86@kernel.org>
Cc: <hpa@zytor.com>
Cc: <mhiramat@kernel.org>
Cc: <peterz@infradead.org>
Cc: <ast@kernel.org>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>