remove admin panel & related code #300

Job	Run time
Apache2 tests on Linux	2m 3s
Apache2 tests on macOS	2m 51s
C++ tests on Linux	1m 37s
C++ tests on macOS	4m 22s
Homebrew packaging tests	7m 6s
Nginx tests on Linux	2m 42s
Nginx dynamic module tests on Linux	1m 15s
Nginx tests on macOS	3m 46s
Nginx dynamic module tests on macOS	1m 18s
Node.js tests on Linux	15s
Node.js tests on macOS	27s
Source packaging tests	25s
Ruby tests on Linux	1m 13s
Passenger Standalone tests on Linux	3m 6s
Ruby tests on macOS	1m 5s
Passenger Standalone tests on macOS	3m 18s
	36m 49s

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