testsuite: Remove needless linux-nat requirement in gdb-sigterm.exp.

That "set debug lin-lwp 1" command even is not needed for the functionality of
this testcase.

This patch does fix a testcase error on aarch64-none-elf.

gdb/testsuite/
2014-03-24  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.base/gdb-sigterm.exp (do_test): Remove "set debug lin-lwp 1".

Message-ID: <20140323165745.GA23830@host2.jankratochvil.net>