kconfig: tests: remove randconfig test for choice in choice

Nesting choice statements does not make any sense.

Commit df8df5e4

 ("usb: get rid of 'choice' for legacy gadget
drivers") got rid of the only usecase.

I will turn it into a syntax error. Remove the test in advance.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>