Remove jl_task_stack_buffer

It has been deprecated and unused since October 2020.

While it doesn't hurt us actively, it is code that needs to be
adjusted for changes to task and TLS handling for no benefit.

src/jl_exported_funcs.inc

@@ -454,7 +454,6 @@
     XX(jl_tagged_gensym) \
     XX(jl_take_buffer) \
     XX(jl_task_get_next) \
-    XX(jl_task_stack_buffer) \
     XX(jl_termios_size) \
     XX(jl_test_cpu_feature) \
     XX(jl_threadid) \

src/julia_gcext.h

@@ -135,15 +135,6 @@ JL_DLLEXPORT int jl_gc_conservative_gc_support_enabled(void);
 // NOTE: Only valid to call from within a GC context.
 JL_DLLEXPORT jl_value_t *jl_gc_internal_obj_base_ptr(void *p);
 
-// Return a non-null pointer to the start of the stack area if the task
-// has an associated stack buffer. In that case, *size will also contain
-// the size of that stack buffer upon return. Also, if task is a thread's
-// current task, that thread's id will be stored in *tid; otherwise,
-// *tid will be set to -1.
-//
-// DEPRECATED: use jl_active_task_stack() instead.
-JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *tid);
-
 // Query the active and total stack range for the given task, and set
 // *active_start and *active_end respectively *total_start and *total_end
 // accordingly. The range for the active part is a best-effort approximation

src/task.c

@@ -326,34 +326,6 @@ void JL_NORETURN jl_finish_task(jl_task_t *ct)
     abort();
 }
 
-JL_DLLEXPORT void *jl_task_stack_buffer(jl_task_t *task, size_t *size, int *ptid)
-{
-    size_t off = 0;
-#ifndef _OS_WINDOWS_
-    jl_ptls_t ptls0 = jl_atomic_load_relaxed(&jl_all_tls_states)[0];
-    if (ptls0->root_task == task) {
-        // See jl_init_root_task(). The root task of the main thread
-        // has its buffer enlarged by an artificial 3000000 bytes, but
-        // that means that the start of the buffer usually points to
-        // inaccessible memory. We need to correct for this.
-        off = ROOT_TASK_STACK_ADJUSTMENT;
-    }
-#endif
-    jl_ptls_t ptls2 = task->ptls;
-    *ptid = -1;
-    if (ptls2) {
-        *ptid = jl_atomic_load_relaxed(&task->tid);
-#ifdef COPY_STACKS
-        if (task->copy_stack) {
-            *size = ptls2->stacksize;
-            return (char *)ptls2->stackbase - *size;
-        }
-#endif
-    }
-    *size = task->bufsz - off;
-    return (void *)((char *)task->stkbuf + off);
-}
-
 JL_DLLEXPORT void jl_active_task_stack(jl_task_t *task,
                                        char **active_start, char **active_end,
                                        char **total_start, char **total_end)