Update/Change `KIGITHUB` variable

Description

• The KIGITHUB variable is obsolete and should be removed

Steps to reproduce

• Install kicad
• Remove ~/.config/kicadnightly/5.99 folder to start with an empty profile
• Start kicad, the variable gets created

KiCad Version

Application: KiCad

Version: 5.99.0-unknown-dbcb1ecdb~104~ubuntu20.10.1, release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.72.0 OpenSSL/1.1.1g zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.8.0 nghttp2/1.41.0 librtmp/2.3

Platform: Linux 5.8.0-2-amd64 x86_64, 64 bit, Little endian, wxGTK, i3, x11

Build Info:
	Date: Oct 25 2020 10:19:24
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCE: 6.9.1
	Curl: 7.68.0
	ngspice: 32
	Compiler: GCC 10.2.0 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
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON