boards: xtensa: kconfig: Remove unused XTENSA_XTSC_INC symbol

Unused since commit 0829ddfe ("kbuild: Removed KBuild").

This symbol is the only thing in boards/xtensa/xt-sim/Kconfig,
which gets included via

    osource "$(BOARD_DIR)/Kconfig"

in boards/Kconfig, so just remove the entire file.

Found with a script.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>