kconfig: remove support for "bool" prompt for choice entries
Since commit fde192511bdb ("kconfig: remove tristate choice support"),
all choice blocks are now boolean. There is no longer a need to specify
the choice type explicitly.
All "bool" prompts in choice entries have been converted to "prompt".
This commit removes support for the "bool" syntax in choice entries.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>