[ffmpeg] Fix build static error

[NancyLi1013]

When building ffmpeg with ffmpeg or ffprobe in static, it will post the error:

There should be no bin\ directory in a static build, but D:\GitHub\vcpkg\packages\ffmpeg_x86-windows-static\bin is present.
There should be no debug\bin\ directory in a static build, but D:\GitHub\vcpkg\packages\ffmpeg_x86-windows-static\debug\bin is present.
If the creation of bin\ and/or debug\bin\ cannot be disabled, use this in the portfile to remove them

    if(VCPKG_LIBRARY_LINKAGE STREQUAL "static")
        file(REMOVE_RECURSE ${CURRENT_PACKAGES_DIR}/bin ${CURRENT_PACKAGES_DIR}/debug/bin)
    endif()

There should be no empty directories in D:\GitHub\vcpkg\packages\ffmpeg_x86-windows-static
The following empty directories were found:

    D:/GitHub/vcpkg/packages/ffmpeg_x86-windows-static/bin
    D:/GitHub/vcpkg/packages/ffmpeg_x86-windows-static/debug/bin

So I remove these folder to fix this issue.

Related issue #9949.

Note: All features have been passed with the following triplets:

• x86-windows
• x64-windows-static (except for opencl, openssl, lzma fixed in PR [ffmpeg] Fix ffmpeg[opencl, openssl, lzma] static build failed #8797 )
• x64-linux (except for opencl, openssl, lzma fixed in PR [ffmpeg] Fix ffmpeg[opencl, openssl, lzma] static build failed #8797 , vpx, x264 only supported on Windows)

[cenit]

wasn't it better to move them to tools, instead of removing them?