Revert "doc: posix: mark posis fd mgmt as supported"

PR #73978 introduced a regression.
Unfortunately this PR cannot be reverted without reverting also
Let's revert both PRs to stabilize main again towards the 3.7 release.

For more details on the issue see
https://github.com/zephyrproject-rtos/zephyr/issues/75205



This reverts commit e9b676a9.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>