target/mips: Remove CPU_NANOMIPS32 definition

nanoMIPS not a CPU, but an ISA. The nanoMIPS ISA is already
defined as ISA_NANOMIPS32.
Remove this incorrect definition and update the single CPU
implementing it, the I7200.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210112210152.2072996-3-f4bug@amsat.org>