i2c: remove dead PMC MSP TWI/SMBus/I2C driver

Commit 1b00767f ("MIPS: Remove PMC MSP71xx platform") removes the
config PMC_MSP in ./arch/mips/Kconfig.

Hence, since then, the corresponding PMC MSP TWI/SMBus/I2C driver is
dead code. Remove this dead driver.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>