st_api: Remove st_context::is_visual_supported.

st_api: Remove st_context::is_visual_supported.

The callback is used by st/vega to check if a visual specifies the
depth/stencil format.  It forces st/vega to be loaded by st/egl to
perform the check.  As noted in EGL spec, the depth/stencil format of a
visual should not affect OpenVG.  It should be better to ignore the
field and always allocate the depth/stencil texture.