Remove `cord_slab_cache` from C API reference

[TarantoolBot]

Related dev. issue(s): GitHub link(s)

Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/fiber/
SME: @ CuriousGeorgiy

Details

The cord_slab_cache needs to be removed from the C API reference of the
fiber module.
Requested by @CuriousGeorgiy in tarantool/tarantool@970ea48.

Definition of done

• cord_slab_cache isn't described in docs