boards: st: remove "- sdhc" tag from supported list

SDHC twister tag indicates that the board supports an SD host controller
driver. Since the ST SDMMC support is provided directly via a disk
driver, SDHC tests are not supported on ST boards. Drop this tag in
order to avoid twister failures running the SDHC tests on these
platforms.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>