ARM: DRA7: configs: Remove obsolete configs

Removing:
uart3_defconfig:
Now uart3 can be selected using menuconfig, removing separate
config for uart mode. Doing uart boot is not straight forward as ROM uses
uart3 as default serial console. In order to boot to prompt, concole in both
u-boot and kernel needs to be changed.

qspiboot_defconfig:
The only advantage of enabling QSPI_BOOT is selecting env in QSPI.
Eventually env needs to be selected by menuconfig so removing
qspiboot_defconfig. qspiboot can be done using dra7xx_evm_defconfig.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>