Kernel: Remove i686 support

Author: https://github.com/supercomputer7
Commit: 5ff318cf3a
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

Kernel/Syscalls/mmap.cpp

@@ -563,14 +563,8 @@ ErrorOr<FlatPtr> Process::sys$allocate_tls(Userspace<char const*> initial_data,
 
         TRY(main_thread->make_thread_specific_region({}));
 
-#if ARCH(I386)
-        auto& tls_descriptor = Processor::current().get_gdt_entry(GDT_SELECTOR_TLS);
-        tls_descriptor.set_base(main_thread->thread_specific_data());
-        tls_descriptor.set_limit(main_thread->thread_specific_region_size());
-#else
         MSR fs_base_msr(MSR_FS_BASE);
         fs_base_msr.set(main_thread->thread_specific_data().get());
-#endif
 
         return m_master_tls_region.unsafe_ptr()->vaddr().get();
     });