(dev/cloud-native#3) SerializeCache - Remove unused, incomplete cache-driver

(dev/cloud-native#3) SerializeCache - Remove unused, incomplete cache-driver

The class `CRM_Utils_Cache_SerializeCache` provides a cache-driver for
storing cache records in the filesystem (under `CIVICRM_TEMPLATE_COMPILEDIR`,
using PHP `serialize()` format). Why remove it?

1.  As we work through cleanup in file management (e.g.  #12843,
   dev/cloud-native#3), having an unnecessary reference to
   CIVICRM_TEMPLATE_COMPILEDIR makes it harder to reason about the system.

2. The class is not used. I'm pretty sure it was added speculatively
   (i.e.  with an aim to try using it some day), but that never came to
   pass.  Grepping `universe`, I cannot find any usages or references to
   `SerializeCache`.

3. The implementation is incomplete -- parameters like `get(..., $default)`
   and `set(..., $ttl)` will generate exceptions if used.