docs/system/target-i386: Remove the sentence about RHEL 7 being supported

docs/system/target-i386: Remove the sentence about RHEL 7 being supported

According to our "Supported build platforms" policy, RHEL 7 is not
supported anymore, so let's remove the related sentence from the x86
documentation.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>