Remove orphaned CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT

After the commit 93d102f0 ("printk: remove safe buffers"),
CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT is no longer useful.  Remove it.

Signed-off-by: Marc Aurèle La France <tsi@tuyoix.net>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
[pmladek@suse.cz: Cleaned up the commit message.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Fixes: 93d102f0 ("printk: remove safe buffers")
Link: https://lore.kernel.org/r/5c19e248-1b6b-330c-7c4c-a824688daefe@tuyoix.net