Remove documentation of UI_UTIL_read_pw*

Remove documentation of UI_UTIL_read_pw*

According to some, a fail-open password verification function is par for
the course for libcrypto. Unfortunately, we have been recommending its use
over similarly named EVP functions after what amounted to a coin toss a
few years back. Luckily enough, no one followed that advice and we can
soon remove this API for good.