tests: drivers: spi: remove useless files and testcases

- with the updates made to the spi_ll_stm32 driver,
we no longer need to configure the 16-bit frame/word size
in the DTS overlay file. The test_spi_word_size_16 testsuite
helps us verify the 16-bit frame mode supported on
the ST platform.

- remove testcases that use thoses files in testcase.yaml

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>