tests: spi_loopback: nxp: remove lpspi async=n cfg

Remove the overlay files for many of the NXP platforms, because they
were doing two things that are no longer relevant:

1) CONFIG_SPI_ASYNC=n -> LPSPI driver used to not work with
   CONFIG_SPI_ASYNC, now it does, so we should test it.
2) CONFIG_SPI_MCUX_LPSPI_DMA=y -> This is redundant with the
   testcase.yaml

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>