.gitattributes: No LFS blobs anymore

LFS was only used for the blueprints documentation. Given that these
docs were drop from the main oniro repo, we can also drop the LFS
configuration.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

.gitattributes

-docs/**/*.png filter=lfs diff=lfs merge=lfs -text
-docs/**/*.jpg filter=lfs diff=lfs merge=lfs -text