MAINT: Remove write to .txt file in to_latex test

A vestige of gh-17960.  Should have been removed before merge.

Author: gfyoung

pandas/tests/io/formats/test_to_latex.py

@@ -400,7 +400,6 @@ def test_to_latex_longtable(self, frame):
 1 &  2 &  b2 \\
 \end{longtable}
 """
-        open("expected.txt", "w").write(withindex_result)
         assert withindex_result == withindex_expected
 
         withoutindex_result = df.to_latex(index=False, longtable=True)