examples: Unify main.c for benchmark and watch panel examples

The two examples have almost identical `main.c` which is mostly a
backbone. The actual applicaton bodies of the two examples are in
`examples/common/benchmark/benchmark_generic_app.c` and
`examples/common/benchmark/benchmark_watch_panel.c` respectively.
Unify `main.c` by only having a common copy of it as `examples/main.c`.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>