Replace zone-has-no-pads DRC test with isolated-copper-fill DRC test

Description

Checking for isolated copper areas is far more discriminating.

Steps to reproduce

1. Create a zone.
2. Set its Island removal strategy to never.
3. Make sure it has no pads, tracks, etc. connecting it.
4. Run DRC.

KiCad Version

Application: KiCad PCB Editor

Version: (6.99.0-1130-g0778bb7803-dirty), debug build

Libraries:
	wxWidgets 3.1.5
	FreeType 2.11.1
	libcurl/7.68.0 OpenSSL/1.1.1d zlib/1.2.11 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0)

Platform: macOS Catalina Version 10.15.7 (Build 19H1615), 64 bit, Little endian, wxMac

Build Info:
	Date: Mar  1 2022 11:33:50
	wxWidgets: 3.1.5 (wchar_t,STL containers)
	Boost: 1.76.0
	OCC: 7.5.1
	Curl: 7.68.0
	ngspice: 31
	Compiler: Clang 11.0.0 with C++ ABI 1002

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE_ADDRESS=OFF
	KICAD_SANITIZE_THREADS=OFF