gh-139227: Remove unused _PyRuntime.imports.pkgcontext (#151490)

The global variable has been replaced by a new thread local variable
"pkgcontext" in Python/import.c.

Author: vstinner

Include/internal/pycore_interp_structs.h

@@ -308,8 +308,6 @@ struct _import_runtime_state {
            Modules are added there and looked up in _imp.find_extension(). */
         struct _Py_hashtable_t *hashtable;
     } extensions;
-    /* Package context -- the full module name for package imports */
-    const char * pkgcontext;
 };
 
 struct _import_state {

Python/import.c

@@ -81,8 +81,6 @@ static struct _inittab *inittab_copy = NULL;
 #define LAST_MODULE_INDEX _PyRuntime.imports.last_module_index
 #define EXTENSIONS _PyRuntime.imports.extensions
 
-#define PKGCONTEXT (_PyRuntime.imports.pkgcontext)
-
 
 /*******************************/
 /* interpreter import state */
@@ -883,7 +881,6 @@ _PyImport_ClearModulesByIndex(PyInterpreterState *interp)
 */
 
 static _Py_thread_local const char *pkgcontext = NULL;
-# undef PKGCONTEXT
 # define PKGCONTEXT pkgcontext
 
 const char *