spec/bsps/arm: Remove the mercury-specific BSP

This BSP has been superceded by the generic BSPs supporting lockstep and
each of the cores in split mode.

Updates #5119

spec/build/bsps/arm/xilinx-zynqmp-rpu/bspmercuryxu5.yml

@@ -1,17 +0,0 @@
-SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
-arch: arm
-bsp: xilinx_zynqmp_mercuryxu5_rpu
-build-type: bsp
-cflags: []
-copyrights:
-- Copyright (C) 2023 Reflex Aerospace GmbH
-cppflags: []
-enabled-by: true
-family: xilinx-zynqmp-rpu
-includes: []
-install: []
-links:
-- role: build-dependency
-  uid: grp
-source: []
-type: build