Don't build host-xz needlessly

Since commit d1f325f5 (xzcat: treat as host prerequisite and build if
needed) host-xz is always built when the host does not have xz installed.
Removed all other host-xz dependencies.

[Peter: don't drop for host-squashfs, as that needs libxz development files]
Cc: Thomas De Schampheleire <patrickdepinguin+buildroot@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>