tracing: Remove __assign_str_len()

tracing: Remove __assign_str_len()

Now that __assign_str() gets the length from the __string() (and
__string_len()) macros, there's no reason to have a separate
__assign_str_len() macro as __assign_str() can get the length of the
string needed.

Also remove __assign_rel_str() although it had no users anyway.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]
Cc: Jeff Layton <[email protected]>
Acked-by: Chuck Lever <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>