safekeeper: remove .partial suffix on the last WAL file.

[arssher]

Reasons:

• it makes pg_waldump usage slightly more cumbersome, forcing to rename file.
• it makes pull_timeline slightly more cumbersome because at any moment source file can be renamed from partial to full.

Leave ability to read .partial files for backward compatibility.

Fixes https://github.com/neondatabase/neon/issues/4459