[GAS][ARM] Fix ARMv8.1 AdvSIMD testism

[GAS][ARM] Fix ARMv8.1 AdvSIMD testism

This test never used to test the output of objdump as the old 'error-output'
check would exit after verifying the output in stdout and stderr from the
assembler.  Given the use of warning_output now, the objdump runs and expects
its output to be verified.  Assuming the correct disassembly of these
instructions is tested elsewhere given we never tested them here, this patch
removes the objdump run.

gas/ChangeLog

2018-10-31  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* testsuite/gas/arm/armv8-a+rdma-warning.d: Remove objdump execution.