Refactor: libcrmcommon: Drop support for unversioned upgrade-enter.xsl

Such a file does not exist. Commit e535c3f4 added support for it as a
fallback when a versioned upgrade-enter stylesheet does not exist.
However, no rationale was given, and it complicates the logic without
clear benefit.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>