doc: remove unneeded .gitattributes

The top-level .gitattributes file contains entries for the Documentation
tree.  Documentation/.gitattributes has not been touched since it was
added in 14f9e12 (Define the project whitespace policy, 2008-02-10).

Signed-off-by: Todd Zullinger <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: tmzullinger