[Pal] Remove SHIM_TCB_USE_GS macro

Pal/src/pal.h

@@ -85,21 +85,12 @@ typedef union pal_handle
 
 #endif /* !IN_PAL */
 
-/* TODO: introduce configuration system in long term and
-         make SHIM_TCB_USE_GS easily configurable without source code
-         modification */
-//#define SHIM_TCB_USE_GS 1
-#undef SHIM_TCB_USE_GS
-
-#if defined(IN_PAL) || defined(SHIM_TCB_USE_GS)
 #define PAL_LIBOS_TCB_SIZE  256
 
 typedef struct pal_tcb {
     struct pal_tcb * self;
     /* uint64_t for alignment */
-#ifdef SHIM_TCB_USE_GS
     uint64_t libos_tcb[(PAL_LIBOS_TCB_SIZE + sizeof(uint64_t) - 1) / sizeof(uint64_t)];
-#endif
     /* data private to PAL implementation follows this struct. */
 } PAL_TCB;
 
@@ -111,7 +102,6 @@ static inline PAL_TCB * pal_get_tcb (void)
              : "i" (offsetof(struct pal_tcb, self)));
     return tcb;
 }
-#endif
 
 typedef struct {
 #ifdef __x86_64__