Kconfig: Remove unused symbols defined only in Kconfig.defconfig files

Turning 'def_bool' in Kconfig.defconfig files into 'default' revealed
three unused symbols (confirmed with 'git grep'). Remove them.

Search for "Kconfig.defconfig" in
https://docs.zephyrproject.org/latest/application/kconfig-tips.html for
an explanation of how def_bool->default can reveal undefined symbols.

Removed unused symbols:

 - SPI_DW_CLOCK_GATE

 - PINMUX_MPS2

 - BOARD_XTENSA

Also remove an assignment to the promptless symbol ALTERA_AVALON_SYSID,
in tests/boards/altera_max10/sysid/prj.conf. Assignments to promptless
symbols have no effect.

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