tracetool: Omit useless QEMU_*_ENABLED() check

SystemTap provides a "semaphore" that can optionally be tested before
executing a trace event.  The purpose of this mechanism is to skip
expensive tracing code when the trace event is disabled.

For example, some applications may have trace events that format or
convert strings for trace events.  This expensive processing should only
be done in the case where the trace event is enabled.

Since QEMU's generated trace events never have such special-purpose
code, there is no reason to add the semaphore check.

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
