Remove refresh_extension_manager

[braingram]

Description of the problem

AsdfFile.schema_info and AsdfFile.info have an argument refresh_extension_manager. This ultimately triggers:

asdf/asdf/_node_info.py

Lines 108 to 117 in dd84c66

	def _get_extension_manager(refresh_extension_manager):
	from ._asdf import AsdfFile, get_config
	from .extension import ExtensionManager
	af = AsdfFile()
	if refresh_extension_manager:
	config = get_config()
	af._extension_manager = ExtensionManager(config.extensions)
	return af.extension_manager

Which loads a fresh default "extension_manager" using default extensions.

This option is entirely unused by downstream and could be deprecated (or perhaps just removed). I am also not sure it needs to be used internally (a quick test at removing it resulted in no test failures).