rget rid of doc/.gitignore due to releaseme

releaseme script doesn't like a .gitignore file under
doc... it ends by adding it as if it were a directory, using
add_subdirectory(.gitignore), with causes this error:

CMake Error at doc/CMakeLists.txt:2 (add_subdirectory):

add_subdirectory given source ".gitignore" which is not an existing
directory.

So, get rid of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>