hw/mips: Remove the 'r4k' machine

We deprecated the support for the 'r4k' machine for the 5.0 release
(commit d32dc61421), which means that our deprecation policy allows
us to drop it in release 5.2. Remove the code.

To repeat the rationale from the deprecation note:
- this virtual machine has no specification
- the Linux kernel dropped support for it 10 years ago

Users are recommended to use the Malta board instead.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <20201102201311.2220005-1-f4bug@amsat.org>

tests/qtest/cdrom-test.c

@@ -217,7 +217,7 @@ int main(int argc, char **argv)
         add_cdrom_param_tests(sparc64machines);
     } else if (!strncmp(arch, "mips64", 6)) {
         const char *mips64machines[] = {
-            "magnum", "malta", "mips", "pica61", NULL
+            "magnum", "malta", "pica61", NULL
         };
         add_cdrom_param_tests(mips64machines);
     } else if (g_str_equal(arch, "arm") || g_str_equal(arch, "aarch64")) {