kconfig: fix set but not used warnings

Remove set but not used variables to fix warnings.

  HOSTCC  scripts/kconfig/gconf.o
/usr/src/lto/scripts/kconfig/gconf.c: In function 'change_sym_value':
/usr/src/lto/scripts/kconfig/gconf.c:833:11: warning: variable 'oldval' set but not used [-Wunused-but-set-variable]
/usr/src/lto/scripts/kconfig/gconf.c: In function 'update_tree':
/usr/src/lto/scripts/kconfig/gconf.c:1281:19: warning: variable 'prop' set but not used [-Wunused-but-set-variable]

Signed-off-by: Peter Foley <pefoley2@verizon.net>
Signed-off-by: Michal Marek <mmarek@suse.cz>