Remove ocaml-system from the list of default compilers used during opam init #4117

Job	Run time
Analyse	7s
Cygwin	4s
Build-macOS (4.14.2)	2m 53s
Hygiene	32s
Build-Linux (4.08.1)	2m 28s
Build-macOS (5.3.0)	3m 36s
Build-Linux (4.09.1)	3m 38s
Test-macOS (4.14.2)	21m 15s
Test-macOS (5.3.0)	30m 35s
Build-Linux (4.10.2)	2m 30s
Build-Linux (4.11.2)	2m 23s
Build-Linux (4.12.1)	2m 51s
Build-Linux (4.13.1)	2m 22s
Build-Linux (5.0.0)	2m 44s
Build-Linux (5.1.1)	2m 26s
Build-Linux (5.2.1)	2m 28s
Build-Linux (4.14.2)	2m 22s
Build-Linux (5.3.0)	3m 23s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	34m 13s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	42m 9s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 25s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	44m 22s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	47m 26s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.3.0)	43m 22s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 5.3.0)	43m 7s
Upgrade-macOS (4.14.2)	1m 47s
Solvers-macOS (z3, 4.14.2)	1m 40s
Solvers-macOS (z3, 5.3.0)	1m 35s
Solvers-macOS (0install, 4.14.2)	1m 38s
Upgrade-macOS (5.3.0)	1m 59s
Solvers-macOS (0install, 5.3.0)	1m 23s
Doc-Linux (4.14.2)	1m 15s
Doc-Linux (5.3.0)	1m 13s
Cold-Linux	5m 12s
Test-Linux (4.14.2)	10m 3s
Solvers-Linux (z3, 4.14.2)	1m 48s
Test-Linux (5.3.0)	11m 14s
Upgrade-Linux (4.14.2)	1m 17s
Solvers-Linux (z3, 5.3.0)	1m 8s
Upgrade-Linux (5.3.0)	2m 13s
Solvers-Linux (0install, 4.14.2)	1m 24s
Solvers-Linux (0install, 5.3.0)	2m 41s
	7h 14m 11s

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