usb: host: remove dead EHCI support for on-chip PMC MSP71xx USB controller

Commit 1b00767f ("MIPS: Remove PMC MSP71xx platform") deletes
./arch/mips/pmcs-msp71xx/Kconfig, including its config  MSP_HAS_USB.

Hence, since then, the corresponding EHCI support for on-chip PMC MSP71xx
USB controller is dead code. Remove this dead driver.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210818071137.22711-2-lukas.bulwahn@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>