libc/picolibc: Remove unused POSIX-ish hooks

While reviewing the dependency between Picolibc POSIX APIs and Zephyr, I
found that the picolibc libc-hooks code copied a bunch of functions from
the newlib version which weren't needed. This required replacing a few
calls to the (now removed) '_write' hook with printk instead.

Signed-off-by: Keith Packard <keithp@keithp.com>