TYPO3 Core

The following patch will be committed in 72 hours if nobody objects

Type: bugfix, cleanup

BT:

Branches: trunk, 4.4

Problem:
The cache manager has some logic to fall back to default frontend and backend definitions if caches are initialized, but not fully defined:
- While these settings might make sense for FLOW3, they are not usefull for v4. In v4 all caches should be defined in TYPO3_CONF_VARS.
- The defaults are of very limited use, it's usually not possible to change a frontend for a cache, and most backends require additional options.
- The defaults are only used in t3lib_cache_manager initialize(), with additionally doesn't use the defined defaultFrontend / defaultBackend settings from TYPO3_CONF_VARS, but own class internal definitions. The method is not used in the core, and it must not be used by an extension.

Solution:
- Remove defaultFrontend / defaultBackend definition from TYPO3_CONF_VARS (not used throughout the core)
- Remove default definitions in t3lib_cache_manager
- Remove default usage in initialize() of t3lib_cache_manager
- Adapt a unit test

Notes:
I don't expect any problems with this as the code is not in use anyway.

(issue imported from #M15388)