[GDBserver] Shorten srv_amd64_linux_xmlfiles

GDBserver now is able to generate target descriptions from features, so
don't need to remember these target description files.

Note that it should be i386/amd64-avx-avx512-linux.xml instead of
i386/amd64-avx-avx512.xml in $srv_amd64_linux_xmlfiles.  This patch
removes it anyway.

gdb/gdbserver:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* configure.srv (srv_amd64_linux_xmlfiles): Remove
	i386/amd64-XXX-linux from it.