libxml2: remove obsolete ptest RDEPENDS

libxml2: remove obsolete ptest RDEPENDS

As part of the libxml2 2.10.3 upgrade[1] the run-ptest was moved from
calling make to running the tests directly, but the make dependency was
not removed.

Also verified that there is no bash usage in the tests, so also remove
the bash dependency.

[1] oe-core ec5f380a142 ("libxml2: upgrade 2.9.14 -> 2.10.3")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>