linux-user: remove configure option for setting uname release

--enable-uname-release was a rather heavyweight hammer, as it allows
providing values less that UNAME_MINIMUM_RELEASE. Also, it affects
all built linux-user targets, which in most cases is not what user
wants.

Now that we have UNAME_MINIMUM_RELEASE for all linux-user platforms,
we can drop --enable-uname-release and the related CONFIG_UNAME_RELEASE
define.

Users can still override the variable with QEMU_UNAME=2.6.32 or -r
command line option. If distributors need to update a minimum version
for a specific target, it can be done by updating UNAME_MINIMUM_RELEASE.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
