build: remove unused CMakeLists.txt

lib/libc/minimal/source/CMakeLists.txt and
lib/libc/minimal/source/stdout/CMakeLists.txt was introduced in
12f8f761 but it is not used by the build system.  CMakeLists.txt in
the parent dir lib/libc/minimal/CMakeLists.txt adds C files to the
target with the lines like:

    ${CMAKE_CURRENT_SOURCE_DIR}/source/stdlib/atoi.c
    ${CMAKE_CURRENT_SOURCE_DIR}/source/stdlib/strtol.c

To make other empty CMakeLists.txt explicit, this commit adds a
comment line to them.

Signed-off-by: Yasushi SHOJI <y-shoji@ispace-inc.com>