examples: Unify app_cfg.h for benchmark and watch panel examples

The `app_cfg.h` headers in the two examples do not contain anything
useful. The only files that include `app_cfg.h` are platform support
code in `examples/common/platform`, but the headers from the two
examples only set macros that are entirely unrelated to platform
support. This is most likely to be unused legacy code that has not been
cleaned up.

Without having any impacts at all on the examples, remove the two
`app_cfg.h` files and create an empty one in the common root directory
of example code, where `main.c` is also located. With only one
configuration, we can build common code once and link both examples
against it, instead of needing two separate builds.

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