remove deprecated export_by_name

[WrRan]

Through exporter_locator was deprecated since nbconvert 5.0,

nbconvert/nbconvert/exporters/exporter_locator.py

Lines 27 to 28 in 2a7419e

	warnings.warn("""`nbconvert.exporters.exporter_locator` is deprecated in favor of `nbconvert.exporters.base` since nbconvert 5.0.""",
	DeprecationWarning)

there still exists an import from it:

nbconvert/nbconvert/exporters/__init__.py

Line 3 in 9470551

from .exporter_locator import export_by_name

This PR trys to fix it. :)

[takluyver]

I'm OK with this. The semantic versioning scheme would say you can only remove things in a major release (like 6.0), but I think it's reasonable to allow rolling removals with a suitable deprecation period. It's been 2 years since 5.0 was released.

[MSeal]

That seems reasonable to me. I was planning on shipping the 5.4.1 release -- should we merge this after or before the patch release you think @takluyver ?

[takluyver]

I wouldn't remove something in a patch release - leave it for 5.5.

[MSeal]

Sounds like we'll need to make some parallel fixes in the notebook server before 5.5 can release with this change fyi :)