Revert "* and #pragma pop_macro("macro_name")"

- pop_macro incorrect with initially undefined macro
- horrible implementation (tcc_open_bf)
- crashes eventually (abuse of Sym->prev_tok)

- the (unrelated) asm_label part is the opposite of a fix
  (Despite of its name this variable has nothing to do with
  the built-in assembler)

This reverts commit 0c8447db79.

tcc.h

@@ -326,7 +326,6 @@
 typedef struct TokenSym {
     struct TokenSym *hash_next;
     struct Sym *sym_define; /* direct pointer to define */
-    struct Sym *sym_define_stack; /* direct pointer to a push_macro stack */
     struct Sym *sym_label; /* direct pointer to label */
     struct Sym *sym_struct; /* direct pointer to structure */
     struct Sym *sym_identifier; /* direct pointer to identifier */