ICU: do not convert locale 'C' to 'en-US-u-va-posix'.

ICU: do not convert locale 'C' to 'en-US-u-va-posix'.

Older versions of ICU canonicalize "C" to "en-US-u-va-posix"; but
starting in ICU version 64, the "C" locale is considered
obsolete. Postgres commit ea1db8ae70 introduced code to always
canonicalize "C" to "en-US-u-va-posix" for consistency and
convenience, but it was deemed too confusing.

This commit removes that code, so that "C" is treated like other ICU
locale names: canonicalization is attempted, and if it fails, the
behavior is controlled by icu_validation_level.

A similar change was previously committed as f7faa9976c, then reverted
due to an ICU-version-dependent test failure. This commit un-reverts
it, omitting the test because we now expect the behavior to depend on
the version of ICU being used.

Discussion: https://postgr.es/m/3a200aca-4672-4b37-fc91-5d198a323503%40eisentraut.org
Discussion: https://postgr.es/m/f83f089ee1e9acd5dbbbf3353294d24e1f196e95[email protected]
Discussion: https://postgr.es/m/37520ec1ae9591f83132f82dbd625f3fc2d69c16[email protected]