gen_compile_commands: remove the warning about too few .cmd files

This warning was useful when users previously needed to manually
build the kernel and run this script.

Now you can simply do 'make compile_commands.json', which updates
all the necessary build artifacts and automatically creates the
compilation database. There is no more worry for a mistake like
"Oh, I forgot to build the kernel".

Now, this warning is rather annoying.

You can create compile_commands.json for an external module:

  $ make M=/path/to/your/external/module compile_commands.json

Then, this warning is displayed since there are usually less than
300 files in a single module.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>