Avoid hardcoding of filesystems supporting CoW

Summary:
The correct way to check for CoW support and conditionally disabling it
for the database is just by setting the NO_COW flag.

The CoW support is not only filesystem dependent, but is affected by
parameters at filesystem creation (XFS reflink) or mount time (BTRFS
nodatacow).

Test Plan:
make
ctest

Reviewers: Baloo, adridg

Reviewed By: adridg

Subscribers: FreeBSD, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12136