[cogl] Remove cogl_{create,destroy}_context from the public API

[cogl] Remove cogl_{create,destroy}_context from the public API

cogl_create_context is dealt with internally when _cogl_get_default context
is called, and cogl_destroy_context is currently never called.

It might be nicer later to get an object back when creating a context so
Cogl can support multiple contexts, so these functions are being removed
from the API until we get a chance to address context management properly.

For now cogl_destroy_context is still exported as _cogl_destroy_context so
Clutter could at least install a library deinit handler to call it.