[prof] remove wrong cast for format string (mono/mono#18477)

[prof] remove wrong cast for format string (mono/mono#18477)

[prof] remove wrong cast for format string

Fixes this warning:
```
  CC       mprof-report.o
mprof-report.c:408:62: warning: format specifies type 'char *' but the argument has type 'guint64' (aka 'unsigned long long') [-Wformat]
                        snprintf (format, sizeof (format), "%s : %%" PRIu64 "\n", (guint64)key_format);
                                                            ~~                    ^~~~~~~~~~~~~~~~~~~
                                                            %llu
/Applications/Xcode112.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/secure/_stdio.h:57:62: note: expanded from macro
      'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str), __VA_ARGS__)
                                                             ^~~~~~~~~~~
```

mistake in refactoring commit https://github.com/mono/mono/commit/mono/mono@07402062541fa9555bef99dbaac84cfb587926cd#diff-c825627ea9ddb3fa753fa3617eaec7c8R408

Commit migrated from https://github.com/mono/mono/commit/73eeda130fceacebec5f1ccbae10478e7fc7c051