printk: remove obsolete dead assignment

printk: remove obsolete dead assignment

Commit 849f3127bb46 ("switch /dev/kmsg to ->write_iter()") refactored
devkmsg_write() and left over a dead assignment on the variable 'len'.

Hence, make clang-analyzer warns:

  kernel/printk/printk.c:744:4: warning: Value stored to 'len' is never read
    [clang-analyzer-deadcode.DeadStores]
                          len -= endp - line;
                          ^

Simply remove this obsolete dead assignment here.

Link: https://lore.kernel.org/r/20201130124915.7573-1-lukas.bulwahn@gmail.com
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>