tests: remove remnants of buffer-count test

The buffer-count test was added in
40c0c3f9 and removed in
4938f93f, but the removal left around
the dependency to EGL headers in weston-test.c.

Removal of those unneeded includes allows us to drop the EGL dependency
completely from weston-test.c build.

For the Meson build this means that there are no dependency('egl')
directives anymore without the user friendly error message.

Signed-off-by: Pekka Paalanen <pq@iki.fi>