ftrace: Remove unused list 'ftrace_direct_funcs'

Commit 8788ca16

 ("ftrace: Remove the legacy _ftrace_direct API")
stopped using 'ftrace_direct_funcs' (and the associated
struct ftrace_direct_func).  Remove them.

Build tested only (on x86-64 with FTRACE and DYNAMIC_FTRACE
enabled)

Link: https://lore.kernel.org/linux-trace-kernel/20240504132303.67538-1-linux@treblig.org

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>