sci-libs/libgeodecomp: Fix file collision with libflatarray

sci-libs/libgeodecomp: Fix file collision with libflatarray

As of commit 948ee21633c0 ("sci-libs/libgeodecomp: added dependency to
libflatarray") libgeodecomp depends on libflatarray which is correct. Therefore,
libgeodecomp uses the system installed libflatarray. However, it also bundles
its own version libflatarray and installs that one unconditionally which results
in the file collision.

Thus, simply don't install the libflatarray files whenever the system one is
used. That resolves the issue.

While here, bump EAPI to 7 and remove visit use flag. VisIt has been removed.

Bug: https://bugs.gentoo.org/688932
Package-Manager: Portage-2.3.79, Repoman-2.3.16
Signed-off-by: Kurt Kanzenbach <kurt@kmk-computers.de>
Closes: https://github.com/gentoo/gentoo/pull/14368
Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>