Revert "tests/tcg/nios2: Re-enable linux-user tests"

nios2 signal tests are broken again:

  retry.py -n 10 -c -- ./qemu-nios2 ./tests/tcg/nios2-linux-user/signals
  Results summary:
  0: 8 times (80.00%), avg time 2.254 (0.00 varience/0.00 deviation)
  -11: 2 times (20.00%), avg time 0.253 (0.00 varience/0.00 deviation)
  Ran command 10 times, 8 passes

This wasn't picked up by CI as we don't have a docker container that
can build QEMU with the nios2 compiler. I don't have time to bisect
the breakage and the target is orphaned anyway so take the easy route
and revert it.

This reverts commit 20e7524f.

Cc: Chris Wulff <crwulff@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-23-alex.bennee@linaro.org>

tests/tcg/nios2/Makefile.target

+# nios2 specific test tweaks
+
+# Currently nios2 signal handling is broken
+run-signals: signals
+	$(call skip-test, $<, "BROKEN")
+run-plugin-signals-with-%:
+	$(call skip-test, $<, "BROKEN")
+run-linux-test: linux-test
+	$(call skip-test, $<, "BROKEN")
+run-plugin-linux-test-with-%:
+	$(call skip-test, $<, "BROKEN")