PyAPI: remove support for importing text blocks as modules

Allowing direct import of text blocks isn't especially useful,
instead add `text.as_module()` script authors can do this
explicitly if it's needed.

Now the text "Register" option executes
instead of loading as a module.

This removes the need to keep track of the current Main,
and C code to override Python's import & reload.

source/blender/python/intern/bpy_operator.c

@@ -37,7 +37,6 @@
 #include "bpy_operator_wrap.h"
 #include "bpy_rna.h" /* for setting arg props only - pyrna_py_to_prop() */
 #include "bpy_capi_utils.h"
-#include "../generic/bpy_internal_import.h"
 #include "../generic/py_capi_utils.h"
 #include "../generic/python_utildefines.h"
 
@@ -320,9 +319,6 @@ static PyObject *pyop_call(PyObject *UNUSED(self), PyObject *args)
    * function corrects bpy.data (internal Main pointer) */
   BPY_modules_update(C);
 
-  /* needed for when WM_OT_read_factory_settings us called from within a script */
-  bpy_import_main_set(CTX_data_main(C));
-
   /* return operator_ret as a bpy enum */
   return pyrna_enum_bitfield_to_py(rna_enum_operator_return_items, operator_ret);
 }