tracing: Remove obsolete PREEMPTIRQ_EVENTS kconfig option

The PREEMPTIRQ_EVENTS option is unused after commit c3bc8fd637a9 ("tracing:
Centralize preemptirq tracepoints and unify their usage"). Remove it.

Note that this option is hazardous as it stands. It enables TRACE_IRQFLAGS
event on non-preempt configurations without the irqsoff tracer enabled.
TRACE_IRQFLAGS as it stands incurs significant overhead on each IRQ
entry/exit. This is because trace_hardirqs_[on|off] does all the per-cpu
manipulations and NMI checks even if tracing is completely disabled for
some insane reason.  For example, netperf running UDP_STREAM on localhost
incurs a 4-6% performance penalty without any tracing if IRQFLAGS is
set. It can be put behind a static brach but even the function entry/exit
costs a little bit.

Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Mel Gorman <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

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