Remove unnecessary debug printing

Callouts in either the IPL or the startup program handle any debug printing that happens early in the system boot (before the serial driver is loaded). These callout routines normally write directly to the registers of the first UART. But before the kernel has initialized, no interrupts are available. So, if the UART FIFO is full, the callouts can't insert a character until another character leaves the FIFO. With a standard UART, a blazingly fast startup can slow to a crawl if you burden the boot process with too many messages.