meson: Don't set -Wall

meson does this for you with its warn levels, so we don't need to set
it ourselves.

Fixes: d1992255
       ("meson: Add build Intel "anv" vulkan driver")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 9c2a95b2)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	meson.build