gh-106320: Remove private _PyDict C API (#107145)

Move private _PyDict functions to the internal C API (pycore_dict.h):

* _PyDict_Contains_KnownHash()
* _PyDict_DebugMallocStats()
* _PyDict_DelItemIf()
* _PyDict_GetItemWithError()
* _PyDict_HasOnlyStringKeys()
* _PyDict_MaybeUntrack()
* _PyDict_MergeEx()

No longer export these functions.