Removed Git attribute merge=union for release notes.

I added this back in 3222fc7 to try
and avoid merge conflicts from concurrent edits to release notes in
different branches. However, in my recent experience, it has caused
more problems than it solves. I have found that when rebasing a branch
that modifies a release note, it can merge sections without an
intermediate blank line, leading to broken reST syntax. Example spotted
in code review:
django#17554 (comment) .

I think it’s better we remove this configuration and deal with merge
conflicts deliberately.

Author: adamchainz

.gitattributes

@@ -4,6 +4,5 @@
 *js text eol=lf
 tests/staticfiles_tests/apps/test/static/test/*txt text eol=lf
 tests/staticfiles_tests/project/documents/test/*txt text eol=lf
-docs/releases/*.txt merge=union
 # Make GitHub syntax-highlight .html files as Django templates
 *.html linguist-language=django