Delete dead code in dict (valkey-io#2014)

After transitioning much usage of dict to the new hashtable, parts of
the dict implementation have become dead code. The following is deleted:

* Dict entry without value
* Dict entry with embedded key
* Pointer bit tricks and other complexity for different kinds of
dictEntry
* Dict two-phase unlink
* Dict type flag to disable incremental rehashing
* Dict stats
* Defrag callbacks defragEntryStartCb and defragEntryFinishCb

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Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>

Author: zuiderkwast