kconfig: remove meaningless prefixes in defconfig files

Since commit e02ee2548afe (kconfig: switch to single .config
configuration), the prefixes in defconfig files such as "+S:",
"+ST:", etc., are meaningless.

This commit was generated by the following command:

  find configs -name '*_defconfig' | xargs sed -i 's/^+*S*T*://'

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/configs/platinum_picon_defconfig b/configs/platinum_picon_defconfig
index c3ca040..c102626 100644
--- a/configs/platinum_picon_defconfig
+++ b/configs/platinum_picon_defconfig
@@ -1,4 +1,4 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6DL"
-+S:CONFIG_ARM=y
-+S:CONFIG_TARGET_PLATINUM_PICON=y
+CONFIG_ARM=y
+CONFIG_TARGET_PLATINUM_PICON=y