powerpc/configs: Drop unnecessary CONFIG_UPROBE_EVENT

In commit 6b0b7551 ("perf/core: Rename CONFIG_[UK]PROBE_EVENT to
CONFIG_[UK]PROBE_EVENTS") it was renamed to CONFIG_UPROBE_EVENTS.

Additionally it's default y, and we have the prerequisites enabled, so
we don't need it in our defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>