cmake: remove "install" target

The deleted "install" target copied only the rimage executable and
left the config files behind. This gave the wrong impression that the
executable is useful without config files.

"Installing" also gave the wrong impression that rimage versions are
somewhat stable and relatively independent of SOF versions: they're
not. In fact there is no such thing as an rimage "version": everyone
should always use the exact rimage _git commit_ from the west manifest
or git submodule. There are no rimage "releases" and no semantic
versioning or anything like it, rimage is effectively part of the SOF
source and build and run directly from its build directory by
practically every SOF developer and SOF CI thanks to

  sof/src/arch/xtensa/CMakeLists.txt#ExternalProject_Add(rimage_ep ...
  sof/scripts/xtensa-build-zephyr.py#def build_rimage()
  sof/zephyr/CMakeLists.txt#set(RIMAGE_CONFIG_PATH ...

etc.

Providing an "install" target greatly increases the chances of
different people and CIs using different rimage versions which is the
last thing we want considering the many significant rimage changes
happening right now, examples:

- zephyrproject-rtos/zephyr#56099
- zephyrproject-rtos/zephyr#54700
- thesofproject/sof#7270
- thesofproject/sof#7304
- thesofproject/sof#7320
- #155

While it's useful for multiple files (e.g.: config files), a CMake
target was always overkill to copy a single file. Someone or some
script who really wants to copy the rimage binary to some other place
that the build directory for some (discouraged) reason _can still do
so_ with a much more basic, simpler and more transparent file copy
command.

Finally, the default "bin" DESTINATION required root access which
is otherwise totally unncessary to build SOF.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

CMakeLists.txt

@@ -64,5 +64,3 @@ target_include_directories(rimage PRIVATE
 	src/include/
 	tomlc99/
 )
-
-install(TARGETS rimage DESTINATION bin)