ftrace: Remove unused pointer ftrace_swapper_pid

ftrace: Remove unused pointer ftrace_swapper_pid

Pointer ftrace_swapper_pid is defined but is never used hence it is
redundant and can be removed. The use of this variable was removed
in commit eeb9a5a34232 ("ftrace: Have set_ftrace_pid use the bitmap
like events do").

Cleans up clang warning:
warning: 'ftrace_swapper_pid' defined but not used [-Wunused-const-variable=]

Link: http://lkml.kernel.org/r/20180809125609.13142-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>