powerpc/configs: Drop no longer needed CONFIG_CRYPTO_GCM

Since commit 00b9cfa3 ("mac80111: Add GCMP and GCMP-256 ciphers")
we no longer need to set CONFIG_CRYPTO_GCM in our defconfigs.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/ppc6xx_defconfig

@@ -1170,7 +1170,6 @@ CONFIG_SECURITY_SELINUX=y
 CONFIG_SECURITY_SELINUX_BOOTPARAM=y
 CONFIG_SECURITY_SELINUX_DISABLE=y
 CONFIG_CRYPTO_TEST=m
-CONFIG_CRYPTO_GCM=m
 CONFIG_CRYPTO_CTS=m
 CONFIG_CRYPTO_LRW=m
 CONFIG_CRYPTO_PCBC=m