x86/tools: Drop duplicate unlikely() definition in insn_decoder_test.c

After commit c104c160 ("Kunit to check the longest symbol length"),
there is a warning when building with clang because there is now a
definition of unlikely from compiler.h in tools/include/linux, which
conflicts with the one in the instruction decoder selftest:

  arch/x86/tools/insn_decoder_test.c:15:9: warning: 'unlikely' macro redefined [-Wmacro-redefined]

Remove the second unlikely() definition, as it is no longer necessary,
clearing up the warning.

Fixes: c104c160 ("Kunit to check the longest symbol length")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250318-x86-decoder-test-fix-unlikely-redef-v1-1-74c84a7bf05b@kernel.org

arch/x86/tools/insn_decoder_test.c

@@ -12,8 +12,6 @@
 #include <stdarg.h>
 #include <linux/kallsyms.h>
 
-#define unlikely(cond) (cond)
-
 #include <asm/insn.h>
 #include <inat.c>
 #include <insn.c>