Staging: lustre: tracefile: Remove wrapper function

Remove the function cfs_trace_free_string_buffer() as it can be replaced
with the standard function kfree().

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Acked-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>