tests: Remove buffer-count

buffer-count was introduced in line with a Mesa change which forced
an earlier block on frame events to try to enforce double-buffering
where available.

The Mesa change has since been reverted (Mesa commit 9ca6711faa), as
this had unpleasant interactions with buffer_age in particular, so this
test is no longer valid.

Additionally, it only worked on backends which initialised EGL (not
headless-backend, where tests generally run), which can be flaky due to
initialisation races. Not only that, but on the DRM backend, we can
legitimately enter triple-buffering due to promoting the surface to a
hardware plane, skipping GPU composition.

In light of all this, just remove the test.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

protocol/weston-test.xml

@@ -64,13 +64,6 @@
       <arg name="x" type="fixed"/>
       <arg name="y" type="fixed"/>
     </event>
-    <request name="get_n_egl_buffers">
-      <!-- causes a n_egl_buffers event to be sent which reports how many
-           buffer objects are live for the client -->
-    </request>
-    <event name="n_egl_buffers">
-      <arg name="n" type="uint"/>
-    </event>
     <request name="capture_screenshot">
       <description summary="records current screen image">
         Records an image of what is currently displayed on a given