Remove undocumented GifImagePlugin._save_netpbm

[akx]

This PR proposes to remove the undocumented (aside from the changelog for PIL 0.2a1 noting "netpbm save hack") GifImagePlugin._save_netpbm function.

[radarhere]

If this function is unimportant enough that we don't need a deprecation period, then I'm also inclined to think it is unimportant enough that we don't need release notes.

[akx]

I've been bitten by libraries quietly removing functions (last in oracle/oci-python-sdk#878).

Augmented the changelog entry to include some history (since I had it at hand), and an alternate recipe. What do you think?

[radarhere]

If you're concerned about removing it without notifying users, then it sounds like a deprecation period is in order. #9186 deprecated the undocumented Image._show() function.

A deprecation period lasts at least a year, so for this PR, it would target Pillow 14.0.0.

[akx]

I'm not overly concerned about a proper deprecation period for an underscore-private feature that was never really documented.

So to avoid a deprecation period, I should remove the changelog entry?

[radarhere]

Looking further,

Pillow/src/PIL/GifImagePlugin.py

Lines 1220 to 1223 in eb3935f

	# Uncomment the following line if you wish to use NETPBM/PBMPLUS
	# instead of the built-in "uncompressed" GIF encoder
	# Image.register_save(GifImageFile.format, _save_netpbm)

seems similar to our rationale for adding a deprecation period to Image._showxv

[akx]

GitHub Code Search seems to imply there is no code using _save_netpbm that is not in one of Pillow's own files).

For _showxv, a similar search today shows only one actual call in code uploaded 2 years ago while _showxv was removed in 4990404, 5 years ago.