target-mips: Remove assignment to a variable which is never used

This assignment causes a compiler warning for compilations with the compiler
option -Wunused-but-set-variable (which is included with -Wextra).

Removing it allows using -Wextra for QEMU code without suppressing too many
extra warnings.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target-mips/op_helper.c

@@ -1735,7 +1735,6 @@ target_ulong helper_evpe(CPUMIPSState *env)
 void helper_fork(target_ulong arg1, target_ulong arg2)
 {
     // arg1 = rt, arg2 = rs
-    arg1 = 0;
     // TODO: store to TC register
 }