"virtual components" still exist in the STEP export dialog; nothing to replace that option

Description

The concept of "virtual component" was removed in v6 but it's still used in the v6.0.1 (and 6.99) STEP export dialog.

This caused trouble in at least one user's workflow; certain components can't be excluded from the export with any dialog option. Therefore this can be seen as a missing feature in and a bug in v6 even if the original option is removed. But I'm not sure how to fix it. Should "Exclude from BOM" and "Exclude from position files" be added as exclusion options in the dialog? And/or Footprint type: Other?

Steps to reproduce

1. File -> Export -> STEP
2. "Ignore virtual components" is in the dialog.

KiCad Version

Application: KiCad PCB Editor

Version: (6.99.0-555-g55b782d66e), release build

Libraries:
	wxWidgets 3.0.5
	FreeType 2.10.4
	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 5.11.0-46-generic x86_64, 64 bit, Little endian, wxGTK, KDE, x11

Build Info:
	Date: Jan 22 2022 00:02:29
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.5.2
	Curl: 7.74.0
	ngspice: 34
	Compiler: GCC 10.3.0 with C++ ABI 1014

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON