sim: mips: delete unused PSIZE define

sim: mips: delete unused PSIZE define

It's unclear what this define is for as it appears to be unused, and
has never been used in the history of the mips sim.  Delete it to tidy
up, and to fix build errors for Windows targets that have a standard
"PSIZE" struct in their system headers.  This doesn't show up yet as
most sim files don't include many system headers, but enabling sockser
code for mingw uncovers the conflict.

Unfortunately the error produced by gcc is inscrutable, but running
it through the preprocessor manually manages to provide a pointer to
the underlying issue.

$ i686-w64-mingw32-gcc ... -c -o dv-sockser.o ../../../../sim/mips/../common/dv-sockser.c
<command-line>: error: expected identifier or '(' before numeric constant
In file included from /usr/i686-w64-mingw32/usr/include/windows.h:71,
                 from /usr/i686-w64-mingw32/usr/include/winsock2.h:23,
                 from ../../gnulib/import/sys/socket.h:684,
                 from ../../gnulib/import/netinet/in.h:43,
                 from ../../../../sim/mips/../common/dv-sockser.c:39:
/usr/i686-w64-mingw32/usr/include/wingdi.h:2934:59: error: unknown type name 'LPSIZE'; did you mean 'LPSIZEL'?
 2934 |   WINGDIAPI WINBOOL WINAPI GetAspectRatioFilterEx(HDC hdc,LPSIZE lpsize);
      |                                                           ^~~~~~
      |                                                           LPSIZEL
...

$ i686-w64-mingw32-gcc ... -E -dD -o dv-sockser.i ../../../../sim/mips/../common/dv-sockser.c
$ i686-w64-mingw32-gcc -c dv-sockser.i
In file included from /usr/i686-w64-mingw32/usr/include/windows.h:69,
                 from /usr/i686-w64-mingw32/usr/include/winsock2.h:23,
                 from ../../gnulib/import/sys/socket.h:684,
                 from ../../gnulib/import/netinet/in.h:43,
                 from ../../../../sim/mips/../common/dv-sockser.c:39:
/usr/i686-w64-mingw32/usr/include/windef.h:104:9: error: expected identifier or '(' before numeric constant
  104 | } SIZE,*PSIZE,*LPSIZE;
      |         ^~