savedefconfig: Remove BR2_DEFCONFIG from saved defconfig file

BR2_DEFCONFIG should not be present in saved defconfig file.

The use case is:
make qemu_arm_versatile
make savedefconfig BR2_DEFCONFIG=my_custom_defconfig

BR2_DEFCONFIG is set in my_custom_defconfig with an absolute path
to qemu_arm_versatile (value present in .config) and set in
my_custom_defconfig as it is different from default mentioned in
config.in (default is BR2_DEFCONFIG from environment).

On savedefconfig recipe, simply remove BR2_DEFCONFIG from generated file

[Peter: fixup typos and use SED as noted by Arnout]
Signed-off-by: Herve Codina <Herve.CODINA@celad.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>