Markers: remove channel name from trace_mark()

*** This is an instrumentation API change ***

Given that UST will gradually move to a scheme where channels are
dynamically associated with markers on a per tracing session basis (and
thus associated dynamically rather than fixed statically), it does not
make sense to specify the "channel name" in addition to the marker name
in the trace_mark() arguments.

API touched:

GET_MARKER()
DEFINE_MARKER()
DEFINE_MARKER_TP()
trace_mark()
_trace_mark()

I'm introducing this API change without changing the underlying
implementation, trying to minimize the impact of API changes by doing
them sooner than later.

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