treewide: replace or remove redundant def_bool in Kconfig files

'def_bool X' is a shorthand for 'bool' plus 'default X'.

'def_bool' is redundant where 'bool' is already present, so 'def_bool X'
can be replaced with 'default X', or removed if X is 'n'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>