Refactoring: remove struct_size from struct lttng_ust_ctx_value

Refactoring: remove struct_size from struct lttng_ust_ctx_value

The struct_size scheme is not appropriate to extend struct
lttng_ust_ctx_value because its type selector and union already
act as a mean to extend it. When new entries are added to the union,
the size of that union may increase, thus shifting the offset of
fields following that union. It is therefore not possible to extend
it by adding fields after the union, thus making the struct_size
scheme irrelevant.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I9e80e0ea87c3df3fe76863ed5343de733210045e