libssh2: Clean up ptest patch/coverage

Martin Jansa spotted patchreview.py reports Malformed Upstream-Status in a
patch in this recipe. The patch is not being applied since there is no ptest
override.

The test in question was also disabled due to an issue with new versions of
openssh.

Add a workaround for the broken test, enable it, drop the broken patch.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e9f2cc08)
Signed-off-by: Steve Sakoman <steve@sakoman.com>