chore(gitignore): drop dead entries

python/paperjam/libpdfium.so pointed at a path that was renamed to
py_src/ long ago, and the Sphinx _build entry is from before the
docs moved to Docusaurus. Both were no-ops.

Author: pratyush618

.gitignore

@@ -33,18 +33,12 @@ CLAUDE.md
 .DS_Store
 Thumbs.db
 
-# Bundled native libraries (binary artifacts)
-python/paperjam/libpdfium.so
-
 # Test fixtures (generated)
 tests/fixtures/large_*.pdf
 
 # Per-session test artifacts (accuracy reports, etc.)
 tests/output/
 
-# Sphinx
-_build
-
 # Lock file (library)
 uv.lock