doc: remove unused readthedocs-sphinx-search

Remove readthedocs-sphinx-search from the RTD build environment because
it seems to be unused and unsupported.

RTD has archived the readthedocs-sphinx-search repo since Apr 2025.
The docs website does not seem to show such function, the search box
in the top-left corner requires a search to be submitted and is not
live.

Fixes: https://tracker.ceph.com/issues/72918
Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>