printk: Remove RTB logging

RTB printk logging was useful on older targets but it is significantly
less so these days. The current setup is also incompatible with other
architectures. Remove the implementation in printk.c. If the feature
is desired later, it can be reimpmented in a more generic way

Change-Id: Ia6e4aeced075048fb7d88747cdae4e6768edd022
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>