[kernel] Clean up syscall code

This is a minor refactor including:
- Removing old commented-out syscall_dispatch function
- Removing IA32_EFER syscall-enable flag setting (this is done by the
  bootloader now)
- Moving much logging from inside process/thread syscalls to the 'task'
  log area, allowing for turning the 'syscall' area down to info by
  default.

src/kernel/syscall.h

@@ -7,7 +7,7 @@ struct cpu_state;
 
 enum class syscall : uint64_t
 {
-#define SYSCALL(id, name, result, ...) name = id,
+#define SYSCALL(id, name, ...) name = id,
 #include "syscalls.inc"
 #undef SYSCALL