CIVICRM_BAO_CACHE_ADAPTER - Remove obsolete option

CIVICRM_BAO_CACHE_ADAPTER - Remove obsolete option

Overview
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The constant `CIVICRM_BAO_CACHE_ADAPTER` was a low-visibility transitional
option added during early 5.x.  It facilitated the deprecation of
`CRM_Core_BAO_Cache` (in favor of `Civi::cache()`).  The option allowed
certain builds/configurations to be tested in a "purer" configuration where
`Civi::cache()` handled all storage.

However, following #17989, the deprecated functions no longer exist.  Hence,
there's no need for transitional mechanism to support them.  The
replacements are now mainline.

Before
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There's some obsolete stuff hanging around.

After
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The obsolete stuff is taking a long walk to the moon.