selftests/timers: remove unused irqcount variable

When building with clang, via:

    make LLVM=1 -C tools/testing/selftest

...clang warns about an unused irqcount variable. clang is correct: the
variable is incremented and then ignored.

Fix this by deleting the irqcount variable.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: John Stultz <jstultz@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>