libzigc/math: Remove straggler rint file in vendored MinGW

Seems like it was missed when implementing `rint`.

This was checked by running:

```
$ ./build/stage3/bin/zig build -p stage4 -Denable-llvm -Dno-lib

$ stage4/bin/zig build test-libc -Dlibc-test-path=<LIBC-TEST-PATH> -Dtest-filter=rint -fqemu -fwasmtime --summary line
Build Summary: 2209/2209 steps succeeded
```

lib/c/math.zig

@@ -49,6 +49,7 @@ comptime {
     }
 
     if (builtin.target.isMinGW() or builtin.target.isMuslLibC()) {
+        symbol(&rint, "rint");
         symbol(&rintf, "rintf");
     }
 
@@ -81,7 +82,6 @@ comptime {
     if (builtin.target.isMuslLibC()) {
         symbol(&copysign, "copysign");
         symbol(&copysignf, "copysignf");
-        symbol(&rint, "rint");
     }
 
     symbol(&copysignl, "copysignl");

lib/libc/mingw/math/arm64/rint.c

@@ -1,12 +0,0 @@
-/**
- * This file has no copyright assigned and is placed in the Public Domain.
- * This file is part of the mingw-w64 runtime package.
- * No warranty is given; refer to the file DISCLAIMER.PD within this package.
- */
-#include <math.h>
-
-double rint (double x) {
-  double retval = 0.0;
-  __asm__ __volatile__ ("frintx %d0, %d1\n\t" : "=w" (retval) : "w" (x));
-  return retval;
-}

src/libs/mingw.zig

@@ -995,7 +995,6 @@ const mingw32_arm32_src = [_][]const u8{
 
 const mingw32_arm64_src = [_][]const u8{
     // mingwex
-    "math" ++ path.sep_str ++ "arm64" ++ path.sep_str ++ "rint.c",
     "math" ++ path.sep_str ++ "arm64" ++ path.sep_str ++ "sincos.S",
 };