printk: Remove obsoleted check for non-existent "user" object

The original check for non-null "user" object was introduced by commit
e11fea92 ("kmsg: export printk records to the /dev/kmsg interface")
when "user" could be NULL if /dev/ksmg was opened for writing.

Subsequent change 750afe7b

 ("printk: add kernel parameter to control
writes to /dev/kmsg") made "user" context required for files opened for
write, but didn't remove now redundant checks for it to be non-NULL.

This patch removes the dead code while preserving the current logic.

Signed-off-by: Stanislav Kinsburskii <stanislav.kinsburski@gmail.com>
CC: Petr Mladek <pmladek@suse.com>
CC: Sergey Senozhatsky <senozhatsky@chromium.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: John Ogness <john.ogness@linutronix.de>
CC: linux-kernel@vger.kernel.org
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/167929571877.2810.9926967619100618792.stgit@skinsburskii.localdomain