Fix security header setting in .htaccess by adding 'onsuccess unset'

The headers might already be set by the system administrator at the http server
level (apache or nginx) for some or all virtualhosts.

Using "always set" in the .htaccess of Nextcloud leads to the situation where
the headers might be set twice (once in the default 'onsuccess' table and once
in the 'always' table)! Which leads to warnings in the admin area.

Adding "onsuccess unset" solves the problem, and forces the header in
the 'onsucess' table to be unset, and the header in the 'always' table to be set.

NOTE: with this change, Nextcloud overrides whatever the system administrator
might have already set

See github issues #16893 #16476 #16938 #18017 and discussion in PR #19002

Signed-off-by: zertrin <zertrin@gmail.com>