Since 97c4382 the `file` package has
been part of stdenv, and no longer needs to be listed explicitly as a
build input.  Let's remove the platform-specific inclusion for mingw64
as suggested by @mehmooda:

  #168413 (comment)

I traced the line removed by this commit through the `git blame`; it
was initially added in this commit (and then shuffled around a few
dozen times by refactorings):

  8b292a1

The commit message indicates that `libpng-1.6.20` was current at the
time.  Although there are [libpng
archives](https://github.com/glennrp/libpng) available in git form,
the older versions don't have their autoconfery vendored in, so I
can't link to them.  Fortunately the relevant bit hasn't changed since
then:

https://github.com/glennrp/libpng/blob/a37d4836519517bdce6cb9d956092321eca3e73b/configure#L5575

```
mingw* | pw32*)
  # Base MSYS/MinGW do not provide the 'file' command needed by
  # func_win32_libid shell function, so use a weaker test based on 'objdump',
  # unless we find 'file', for example because we are cross-compiling.
  if ( file / ) >/dev/null 2>&1; then
    lt_cv_deplibs_check_method='file_magic ^x86 archive import|^x86 DLL'
    lt_cv_file_magic_cmd='func_win32_libid'
  else
    # Keep this pattern in sync with the one in func_win32_libid.
    lt_cv_deplibs_check_method='file_magic file format (pei*-i386(.*architecture: i386)?|pe-arm-wince|pe-x86-64)'
    lt_cv_file_magic_cmd='$OBJDUMP -f'
  fi
  ;;
```