target/nios2: Remove the deprecated Nios II target

The Nios II target is deprecated since v8.2 in commit 9997771bc1
("target/nios2: Deprecate the Nios II architecture").

Remove:
- Buildsys / CI infra
- User emulation
- System emulation (10m50-ghrd & nios2-generic-nommu machines)
- Tests

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Message-Id: <20240327144806.11319-3-philmd@linaro.org>

scripts/probe-gdb-support.py

@@ -37,7 +37,6 @@ mappings = {
     "m68k" : "m68k",
     "MicroBlaze" : "microblaze",
     "mips:isa64" : ["mips64", "mips64el"],
-    "nios2" : "nios2",
     "or1k" : "or1k",
     "powerpc:common" : "ppc",
     "powerpc:common64" : ["ppc64", "ppc64le"],