target/mips: Drop pread and pwrite syscalls from semihosting

target/mips: Drop pread and pwrite syscalls from semihosting

We don't implement it with _WIN32 hosts, and the syscalls
are missing from the gdb remote file i/o interface.
Since we can't implement them universally, drop them.

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