Simplify the .gitattributes file (#5268)

* Removes all per-file attributes, many of which were incorrectly
  removing the text attribute from text/source files
* Removes all svneol attributes (not sure where those came from, but
  they don't appear in the git attributes docs)
* Marks *.h5 and certain test files as binary
* Removes the !eol attribute, which is unnecessary given text=auto
  and default git text handling