aarch64: Remove duplicate system register entries

There is a lot of overlap between the ETM and ETE system registers,
so some registers were listed twice.

Already tested by etm.[sd] and ete.[sd].

opcodes/
	* aarch64-opc.c (aarch64_sys_regs): Combine ETE and ETM blocks
	and remove redundant entries.

gas/
	* testsuite/gas/aarch64/etm.s: Remove duplicated test.
	* testsuite/gas/aarch64/etm.d: Update accordingly.