Remove MANIFEST which was used by ancient extmk.rb #6

Job	Run time
ruby-versions / Generate Ruby versions	2s
test (2.5, ubuntu-latest)	32s
test (2.5, macos-latest)	55s
test (2.5, windows-latest)	2m 56s
test (2.6, ubuntu-latest)	42s
test (2.6, macos-latest)	59s
test (2.6, windows-latest)	2m 42s
test (2.7, ubuntu-latest)	36s
test (2.7, macos-latest)	1m 6s
test (2.7, windows-latest)	2m 42s
test (3.0, ubuntu-latest)	25s
test (3.0, macos-latest)	34s
test (3.0, windows-latest)	2m 51s
test (3.1, ubuntu-latest)	20s
test (3.1, macos-latest)	32s
test (3.1, windows-latest)	1m 56s
test (3.2, ubuntu-latest)	36s
test (3.2, macos-latest)	37s
test (3.2, windows-latest)	3m 36s
test (head, ubuntu-latest)	26s
test (head, macos-latest)	48s
test (head, windows-latest)	1m 43s
test (jruby, ubuntu-latest)	5m 38s
test (jruby-head, ubuntu-latest)	5m 32s
test (truffleruby, ubuntu-latest)	2m 25s
test (truffleruby-head, ubuntu-latest)	4m 18s
	45m 29s

Provide feedback