LibX86: Remove i686 support

Author: https://github.com/supercomputer7
Commit: e1ee8f89f2
Pull-request: https://github.com/SerenityOS/serenity/pull/15467
Issue: https://github.com/SerenityOS/serenity/issues/15444
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/fuel-pcbox

Userland/Libraries/LibX86/Disassembler.h

@@ -22,15 +22,11 @@ public:
     {
         if (!m_stream.can_read())
             return {};
-#if ARCH(I386)
+#if ARCH(X86_64)
-        return Instruction::from_stream(m_stream, ProcessorMode::Protected);
-#else
-#    if ARCH(X86_64)
         return Instruction::from_stream(m_stream, ProcessorMode::Long);
-#    else
+#else
         dbgln("Unsupported platform");
         return {};
-#    endif
 #endif
     }