Collate handling of LTTNG_ENABLE_EVENT_WITH_FILTER

Because the handling of LTTNG_ENABLE_EVENT_WITH_EXCLUSION can
now handle commands of type LTTNG_ENABLE_EVENT_WITH_FILTER,
there's no reason to have the same code appear twice.

Remove the code for LTTNG_ENABLE_EVENT_WITH_FILTER and use the
same code as is used for LTTNG_ENABLE_EVENT_WITH_EXCLUSION.