docs: remove dirname.c from THIRDPARTY file (#56413)

- `dirname.c` was removed by
c2cec7a

Author: inkydragon

THIRDPARTY.md

@@ -6,7 +6,6 @@ for exceptions.
 - [crc32c.c](https://stackoverflow.com/questions/17645167/implementing-sse-4-2s-crc32c-in-software) (CRC-32c checksum code by Mark Adler) [[ZLib](https://opensource.org/licenses/Zlib)].
 - [LDC](https://github.com/ldc-developers/ldc/blob/master/LICENSE) (for ccall/cfunction ABI definitions) [BSD-3]. The portion of code that Julia uses from LDC is [BSD-3] licensed.
 - [LLVM](https://releases.llvm.org/3.9.0/LICENSE.TXT) (for parts of src/disasm.cpp) [UIUC]
-- [MINGW](https://sourceforge.net/p/mingw/mingw-org-wsl/ci/legacy/tree/mingwrt/mingwex/dirname.c) (for dirname implementation on Windows) [MIT]
 - [NetBSD](https://www.netbsd.org/about/redistribution.html) (for setjmp, longjmp, and strptime implementations on Windows) [BSD-3]
 - [Python](https://docs.python.org/3/license.html) (for strtod implementation on Windows) [PSF]
 - [FEMTOLISP](https://github.com/JeffBezanson/femtolisp) [BSD-3]