perf: remove tests from target

The perf tool installed test files in
    output/target/usr/libexec/perf-core/tests/
which amounted to about 30+K.

Since they are not needed for normal perf operation, remove them.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>