Unchecking the Exclude from board property of symbol doesn't have effect after footprint has already been added to board

Description

"Exclude from board" should remove the already placed footprint from the board if checked after the footprint has been added by a former update. Now it doesn't have effect on an existing footprint.

Steps to reproduce

1. Add a symbol, attach a footprint, update PCB normally.
2. Check "Exclude from board" property of the schematic symbol.
3. Update the PCB again.
4. The footprint stays in the PCB.

KiCad Version

Application: KiCad Schematic Editor

Version: (5.99.0-10557-g64afa3a42e), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.74.0 OpenSSL/1.1.1j zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.5/openssl/zlib nghttp2/1.43.0 librtmp/2.3

Platform: Linux 5.11.0-16-generic x86_64, 64 bit, Little endian, wxGTK, KDE, x11

Build Info:
	Date: May 12 2021 18:20:09
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.5.1
	Curl: 7.74.0
	ngspice: 34
	Compiler: GCC 10.3.0 with C++ ABI 1014

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
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON