Remove deprecated "N" format for Decimal

[donbarbos]

Deprecated since Python 3.13; the previous deprecation discussion can be found in #89902

Based on the discussion there, it seems that format was intended to be removed in Python 3.15.

If there is a consensus that it would be better to wait a bit longer (in line with PEP 387), I would suggest adding it to the “Pending Removal” section, so that the planned removal is clearly tracked and not accidentally overlooked.

Linked PRs

• gh-143331: Schedule to remove format "N" for Decimal #143372
• [3.14] gh-143331: Schedule to remove format "N" for Decimal (GH-143372) #143476
• [3.13] gh-143331: Schedule to remove format "N" for Decimal (GH-143372) #143477

[picnixz]

If it's deprecated since 3.13, we should wait until 3.18 or even later. We don't need to rush this removal as it was only supported in the C version.