cogl/tests: Remove euler test

Next commits, and this patchset in general, will make this patchset
obsolete, since it'll only test graphene types against each other.
If at all useful, the Euler test should be moved to graphene.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1439

cogl/tests/conform/test-declarations.h

@@ -44,7 +44,6 @@ void test_pipeline_shader_state (void);
 void test_gles2_context (void);
 void test_gles2_context_fbo (void);
 void test_gles2_context_copy_tex_image (void);
-void test_euler (void);
 void test_color_hsl (void);
 void test_fence (void);
 void test_texture_no_allocate (void);