Bus aliases are not deleted in sub files

Description

When bus alias is defined in sub-schematic it cannot be deleted. It is deleted from the list but re-apears on reopening the Bus alias dialog.

Steps to reproduce

1. Open Multi sheet schematic
2. Enter any sub-sheet
3. Open bus alias dialog and add one bus with one member.
4. Close dialog
5. Open bus alias dialog
6. delete bus & close
7. Open bus alias - the bus is still there.

KiCad Version

Application: Eeschema
Version: 5.99.0-unknown-26803f5~101~ubuntu19.10.1, release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.65.3 OpenSSL/1.1.1c zlib/1.2.11 libidn2/2.2.0 libpsl/0.20.2 (+libidn2/2.0.5) libssh/0.9.0/openssl/zlib nghttp2/1.39.2 librtmp/2.3
Platform: Linux 5.3.0-51-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    Build date: May  7 2020 18:35:40
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.65.3
    Compiler: GCC 9.2.1 with C++ ABI 1013

Build settings:
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON