Move fci_l1c_fdhsi reader from pending to full deprecation

[ameraner]

Feature Request

Is your feature request related to a problem? Please describe.
The fci_l1c_fdhsi reader has been renamed to fci_l1c_nc in #1712. It has been temporarily added to the PENDING_OLD_READER_NAMES, such that users using the old name get just a FutureWarning and have time for a gentle transition until the old reader name is fully deprecated.

This issue is a reminder to send fci_l1c_fdhsi to the OLD_READER_NAMES once enough notice time has passed, and later on remove it also from there.

Describe any changes to existing user workflow
When the reader will be moved to the OLD_READER_NAMES, users will get a hard ValueError and will not be able to use the old name any further.

Additional context
To proceed with the deprecation, it's probably best to wait at least until EUMETSAT updates the reader name in the Pytroll section of its Product User Guide for FCI. See link in https://www.eumetsat.int/mtg-test-data (current version is: https://www-cdn.eumetsat.int/files/2020-07/pdf_mtg_fci_l1_pug.pdf (V1J)).